Output external RSS feed from URL in field value when using WP Job Manager Field Editor (untested)

functions.php

<?php
// ^ the <?php above should only be in your functions.php file ONCE, at the top

// The filter is field_editor_output_as_value_METAKEY
// where you need to replace METAKEY with the actual meta key you want to filter the output for

// This example is used for the "job_external_rss_feed" meta key
add_filter( 'field_editor_output_as_value_job_external_rss_feed', 'smyles_output_rss_feed_from_field_value' );

// It is STRONGLY recommended that you set this field type to an HTML5 URL field, to force validation of the URL

/**
 * Output list formatted RSS feed from external URL
 *
 * @author             Myles McNamara
 * @url                https://plugins.smyl.es
 *
 * @param    $value    The value before it is output on the listing
 */
function smyles_output_rss_feed_from_field_value( $value ) {

	if ( empty( $value ) ) {
		return $value;
	}

	// Return empty string if value entered by user is not a valid URL
	if ( filter_var( $value, FILTER_VALIDATE_URL ) === false ) {
		return '';
	}
	
	if ( ! function_exists( 'fetch_feed' ) ) {
		include_once( ABSPATH . WPINC . '/feed.php' );
	}

	$maxitems = 5; // Max items to display

	// RSS feed code from Stackoverflow post
	// @see https://wordpress.stackexchange.com/questions/99584/display-external-rss-feed-on-pages

	// Updated based on code from WordPress Codex
	// @see https://developer.wordpress.org/reference/functions/fetch_feed/

	// Get a SimplePie feed object from the specified feed source.
	$feed = fetch_feed( esc_url( $value ) );

	if ( is_wp_error( $feed ) || ! $feed->get_item_quantity() ) {
		return ''; // Return empty string if error or not items, you could also output custom message here too for when feeds are down
	}

	$rss_items = $feed->get_items( 0, $maxitems );
	$lis       = array();

	if ( empty( $rss_items ) ) {
		return ''; // If for some weird reason no items returned from get_items() return empty string (just like above)
	}

	foreach ( (array) $rss_items as $item ) {
		if ( '' === $title = esc_attr( strip_tags( $item->get_title() ) ) ) {
			$title = __( 'Untitled' );
		}

		$lis[] = sprintf(
			'<a href="%1$s" target="_blank">%2$s</a>',
			esc_url( strip_tags( $item->get_permalink() ) ),
			$title
		);
	}

	return '<ul class="feed-list"><li>' . join( '</li><li>', $lis ) . '</ul>';
}