Looking at your specific example … You probably want to use a shortcode. Something like:
add_shortcode('hide_from_summary','wpse_hide_stuff_here');
function wpse_hide_stuff_here($atts,$content){
global $post; // gives access to the post, so you can determine mode
// don't know if you'll want/need $atts, so ... generic stuff here
$atts = shortcode_atts( array(
'foo' => 'no foo'
), $atts, 'bartag' );
// default output of this shortcode is blank (hidden)
$out="";
//Only show your content if ... you are viewing the post by itself
if (is_a($post, 'WP_Post') && is_single($post)) $out = $content;
return $out;
}
You can add this shortcode to your functions.php or a plugin. Then, apply it in the following way.
[hide_from_summary]
<p>THIS TEXT ONLY SHOWS IN SINGLE POST MODE SOMEHOW</p>
[/hide_from_summary]