takes a custom field named enclosure that holds a string of data with the first element being the URL of an image and make it a featured image for the post

fwp-fi-example.php

add_action( 'publish_post', 'fwp_fi_update', 10, 1 );

function fwp_fi_update ($post_id){

   if (get_post_meta( $post_id, 'enclosure', true )) {
      $messy_data = get_post_meta( $post_id, 'enclosure', true);
      $array_data = explode(PHP_EOL, $messy_data);
      $img_url = $array_data[0];
      $img_id = fwp_fi_add_img($img_url, $post_id);
      fwp_fi_set_featured_img($post_id, $img_id);
   }
}

function fwp_fi_add_img($img_url, $post_id){
   $desc = 'I do not feel like figuring this out right now.';
   $attach_id = media_sideload_image($img_url, $post_id, $desc, 'id');
   return $attach_id;
}


function fwp_fi_set_featured_img($post_id, $img_id){
   set_post_thumbnail( $post_id, $img_id );
}