This is more PHP than WordPress, but wp_get_post_terms
returns a numerically indexed array, so all you need is basic PHP array syntax to grab the first item.
$terms = wp_get_post_terms($post->ID, 'custom_cat');
print_r($terms[0]->name);
You don’t specify which item you want. This will get you last item:
$terms = array_pop($terms);
print_r($terms);