Hi there,
you can use the_excerpt filter:
https://developer.wordpress.org/reference/hooks/the_excerpt/
As an example:
add_filter( 'the_excerpt', function( $excerpt ) {
$my_custom_excerpt = get_the_custom_value;
if ( is_search() && !empty( $my_custom_excerpt )) {
$excerpt = $my_custom_excerpt;
}
return $excerpt;
} );
Of course this:
$my_custom_excerpt = get_the_custom_value;
will need updating to get the value you require. You may need to ask Rank Math how to get their description whilst in the loop. Happy to help if they can provide that bit of info