WP: get sub menu item from specific menu location.

functions.php

<?php


$items = wp_get_nav_menu_items( 'top' );

if ( $items ) {
  echo '<ul class="menu">';
  foreach ( $items as $index => $item ) {
    if ( ! empty( $item->menu_item_parent ) ) {
      echo '<li class="menu-item' . implode( ' ', $item->classes ) . '">';
      echo '<a href="' . $item->url . '" aria-current="page">';
      echo $item->title;echo '</a>';
      echo '</li>';
    }
  }
	echo '</ul>';
}