After Lots of Research and Testing, I got the answer.
the_excerpt and other custom function are not working because there is no any content tag On feed xml format.
You just need to add this code instead of the_excerpt
<p><?php echo esc_html( $item->get_description() ); ?></p>
More you can Visit https://wordpress.org/support/topic-tag/get_description/
The Full code may you try.
<?php
$feed = fetch_feed( 'http://example.com/rss/feed/goes/here' );
if (!is_wp_error( $feed ) ) : // Checks that the object is created correctly
// Figure out how many total items there are, but limit it to 10.
$maxitems = $feed->get_item_quantity(10);
// Build an array of all the items, starting with element 0 (first element).
$rss_items = $feed->get_items(0, $maxitems);
endif;
?>
<?php if ($maxitems == 0) echo '<li>No items.</li>';
else
// Loop through each feed item and display each item as a hyperlink.
foreach ( $rss_items as $item ) : ?>
<div>
<?php
//Use regular expressions to find all images in the post
$output = preg_match_all('/<img.+src=[\'"]([^\'"]+)[\'"].*>/i', $item->get_content(), $matches);
//Grab the first image to use as a thumbnail
$first_img = $matches [1][0];
//If an image exists, display it
if($first_img) {echo '<img src="'.$first_img.'" alt="'.$item->get_title().'" />';}
?>
<h5><a href="<?php echo esc_url( $item->get_permalink() ); ?>"
title="<?php echo "Posted '.$item->get_date('j F Y | g:i a'); ?>'>
<?php echo esc_html( $item->get_title() ); ?></a></h5>
<p><?php echo $item->get_date('n/d/Y'); ?></p>
</div>
<p><?php echo esc_html( $item->get_description() ); ?></p>
<?php endforeach; ?>