Parent for a single (single.php)

by

For a hierarchical post type, you can use $post->post_parent and get_permalink(), perhaps like so:

<?php
global $post;
$parent_permalink = get_permalink( $post->post_parent );
?>
<a href="https://wordpress.stackexchange.com/questions/99483/<?php echo $parent_permalink; ?>">Parent Post</a>