I'm thoroughly confused with this thread. Are we talking about utilizing the clinch under muay thai rules or boxing? Because an underhook is drastically different in MT than it is in boxing when referencing offensive/defensive positioning.
Specifically, if you're referencing boxing than "Giving up the underhook to punch over it"..... well, sure you can. But as far as positioning is concerned in boxing (generally speaking) the one letting go of the underhook is defensively more vulnerable to having a punch come over the top of it. I.E: if boxing, you've got 2 weapons and if ones got an underhook and the other ones crossfacing then neither of them are offensive or defensive. In short, all your opponent need do is swing!