Allow iframe in the WordPress post excerpt.

allow-html-tags-in-post-excerpt.php

<?php // don't copy this line in your code.

function your_prefix_custom_excerpt($text) {
	$raw_excerpt = $text;

	if ( '' == $text ) {
	    //Retrieve the post content. 
	    $text = get_the_content('');
	 
	    //Delete all shortcode tags from the content. 
	    $text = strip_shortcodes( $text );
	 
	    $text = apply_filters('the_content', $text);
	    $text = str_replace(']]>', ']]&gt;', $text);
	     
	    $allowed_tags = '<div>,<iframe>'; // Add comma separated tags that should be displayed in the excerpt.
	    $text = strip_tags($text, $allowed_tags);
	     
	    $excerpt_word_count = 55; // Ccustom excerpt work count.
	    $excerpt_length = apply_filters('excerpt_length', $excerpt_word_count); 
	     
	    $excerpt_end = '[...]'; // Modify this if you want to change the excerpt_more tag.
	    $excerpt_more = apply_filters('excerpt_more', ' ' . $excerpt_end);
	     
	    $words = preg_split("/[\n\r\t ]+/", $text, $excerpt_length + 1, PREG_SPLIT_NO_EMPTY);
	    if ( count($words) > $excerpt_length ) {
	        array_pop($words);
	        $text = implode(' ', $words);
	        $text = $text . $excerpt_more;
	    } else {
	        $text = implode(' ', $words);
	    }
	}

	return apply_filters('wp_trim_excerpt', $text, $raw_excerpt);
}

remove_filter('get_the_excerpt', 'wp_trim_excerpt');
add_filter('get_the_excerpt', 'your_prefix_custom_excerpt');

Change line #16 with HTML tags which should appear in the excerpt.