Get the excerpt for a WP_Post by post ID.

wp-get-excerpt-by-id.php

<?php

/**
 * Get the excerpt for a WP_Post by post ID.
 * 
 * @param int $post_id
 */
function get_excerpt_by_id( $post_id = 0 ) {
	global $post;
	$save_post = $post;
	$post = get_post( $post_id );
	setup_postdata( $post );
	$excerpt = get_the_excerpt();
	$post = $save_post;
	wp_reset_postdata( $post );
	return $excerpt;
}