A wordpress shortcode to display the current post truncated content. I did this shortcode to use with Paid Member Subscriptions in the first place...

truncated_content_shortcode.css

/* truncated content styles : add this to the child theme style.css */
div.truncated_content {
	position: relative;
}
div.truncated_content:before {
	content: '';
	position: absolute;
	width: 100%;
	height: 100px;
	height: 100%;
	bottom: 0;
	cursor: text;
	/* "transparent" only works here because == rgba(0,0,0,0) */
	background-image: -webkit-gradient( linear, left top, left bottom, color-stop( 0, transparent ), color-stop( 1, white ) );
	background-image: -webkit-linear-gradient( top, transparent, white );
	background-image:    -moz-linear-gradient( top, transparent, white );
	background-image:     -ms-linear-gradient( top, transparent, white );
	background-image:      -o-linear-gradient( top, transparent, white );
	background-image:         linear-gradient( to bottom, transparent, white );
}

truncated_content_shortcode.php

<?php
/**
 * Plugin Name: Post truncated content
 * Description: Shortcode to display current post truncated content.
 * Version: 1.0
 * Author: Sébastien Méric
 * License:     GNU General Public License v2.0 or later
 * License URI: http://www.gnu.org/licenses/gpl-2.0.html
 *
 * Text Domain: sm-tcsc
 * Domain Path: /languages/
 *
 * This program is free software; you can redistribute it and/or modify it under the terms of the GNU
 * General Public License as published by the Free Software Foundation; either version 2 of the License,
 * or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without
 * even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 *
 * You should have received a copy of the GNU General Public License along with this program; if not, write
 * to the Free Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
 */

if ( ! defined( 'ABSPATH' ) )
	exit;

if ( ! class_exists( 'SM_Truncated_Content_Shortcode' ) ) {

	class SM_Truncated_Content_Shortcode {

		private static $instance;

		/**
		 * Main Instance
		 *
		 * Ensures that only one instance exists in memory at any one
		 * time. Also prevents needing to define globals all over the place.
		 *
		 * @since 1.0
		 *
		 */
		public static function instance() {
			if ( ! isset ( self::$instance ) ) {
				self::$instance = new self;
			}

			return self::$instance;
		}

		/**
		 * Start your engines
		 *
		 * @since 1.0
		 *
		 * @return void
		 */
		public function __construct() {
			$this->setup_globals();
			$this->setup_actions();
			$this->load_textdomain();
		}

		/**
		 * Globals
		 *
		 * @since 1.0
		 *
		 * @return void
		 */
		private function setup_globals() {
			// paths
			$this->file       = __FILE__;
			$this->basename   = apply_filters( 'sm_tcsc_plugin_basenname', plugin_basename( $this->file ) );
			$this->plugin_dir = apply_filters( 'sm_tcsc_plugin_dir_path',  plugin_dir_path( $this->file ) );
			$this->plugin_url = apply_filters( 'sm_tcsc_plugin_dir_url',   plugin_dir_url ( $this->file ) );
		}

		/**
		 * Setup the default hooks and actions
		 *
		 * @since 1.0
		 *
		 * @return void
		 */
		private function setup_actions() {
			// truncated content shortcode
			add_shortcode( 'truncated_content', array( $this, 'truncated_content' ) );

			// action
			//add_action( 'hook_name', array( $this, 'function_name' ), 10, 2 );
			// filter
			//add_filter( 'hook_name', array( $this, 'function_name' ) );

			do_action( 'sm_tcsc_setup_actions' );
		}

		/**
		 * Loads the plugin language files
		 *
		 * @access public
		 * @since 1.0
		 * @return void
		 */
		public function load_textdomain() {
			// Set filter for plugin's languages directory
			$lang_dir = dirname( plugin_basename( $this->file ) ) . '/languages/';
			$lang_dir = apply_filters( 'sm_tcsc_languages_directory', $lang_dir );

			// Traditional WordPress plugin locale filter
			$locale = apply_filters( 'plugin_locale', get_locale(), 'sm-tcsc' );
			$mofile = sprintf( '%1$s-%2$s.mo', 'sm-tcsc', $locale );

			// Setup paths to current locale file
			$mofile_local  = $lang_dir . $mofile;
			$mofile_global = WP_LANG_DIR . '/sm-tcsc/' . $mofile;

			if ( file_exists( $mofile_global ) ) {
				load_textdomain( 'sm-tcsc', $mofile_global );
			}
			elseif ( file_exists( $mofile_local ) ) {
				load_textdomain( 'sm-tcsc', $mofile_local );
			}
			else {
				// Load the default language files
				load_plugin_textdomain( 'sm-tcsc', false, $lang_dir );
			}
		}

		/**
		 * Shortcode to display a post truncated content
		 *
		 * @access public
		 * @since 1.0
		 * @param  array  $atts Shortcode attributes
		 * @return string       Shortcode rendered
		 */
		public function truncated_content( $atts ) {
			// Attributes
			extract( shortcode_atts(
				array(
					'size' => '55',
					'more' => '&hellip;',
				), $atts )
			);

			if ( is_singular() ) {
				global $post;

				$excerpt = $this->wp_trim_words_retain_formatting( $post->post_content, $size, $more );
				$excerpt = balanceTags( $excerpt, true );
				$excerpt = wpautop( $excerpt );
			}

			return '<div class="truncated_content">' . $excerpt . '</div>';

		}

		/**
		 * A variant of wp_trim_words that keeps html formating.
		 *
		 * @access public
		 * @since 1.0
		 * @param  string $text      The original text.
		 * @param  int    $num_words The number of words to trim the text to. Default 55.
		 * @param  string $more      An optional string to append to the end of the trimmed text. Default &hellip;
		 * @return string            The trimmed text.
		 */
		public function wp_trim_words_retain_formatting( $text, $num_words = 55, $more = null ) {
			if ( null === $more )
				$more = __( '&hellip;' );
			$original_text = $text;
			/* translators: If your word count is based on single characters (East Asian characters),
			   enter 'characters'. Otherwise, enter 'words'. Do not translate into your own language. */
			if ( 'characters' == _x( 'words', 'word count: words or characters?' ) && preg_match( '/^utf\-?8$/i', get_option( 'blog_charset' ) ) ) {
				$text = trim( preg_replace( "/[\n\r\t ]+/", ' ', $text ), ' ' );
				preg_match_all( '/./u', $text, $words_array );
				$words_array = array_slice( $words_array[0], 0, $num_words + 1 );
				$sep = '';
			}
			else {
				$words_array = preg_split( "/[\n\r\t ]+/", $text, $num_words + 1, PREG_SPLIT_NO_EMPTY );
				$sep = ' ';
			}
			if ( count( $words_array ) > $num_words ) {
				array_pop( $words_array );
				$text = implode( $sep, $words_array );
				$text = $text . $more;
			}
			else {
				$text = implode( $sep, $words_array );
			}
			/**
			 * Filter the text content after words have been trimmed.
			 *
			 * @since 1.0
			 *
			 * @param string $text          The trimmed text.
			 * @param int    $num_words     The number of words to trim the text to. Default 5.
			 * @param string $more          An optional string to append to the end of the trimmed text, e.g. &hellip;.
			 * @param string $original_text The text before it was trimmed.
			 */
			return apply_filters( 'wp_trim_words', $text, $num_words, $more, $original_text );
		}

	}

	/**
	 * Get everything running
	 *
	 * @since 1.0
	 *
	 * @return void
	 */
	add_action( 'plugins_loaded', 'sm_tcsc' );
	function sm_tcsc() {
		$sm_tcsc = new SM_Truncated_Content_Shortcode();
	}
}