Relaxing during Muay Thai Clinch?

Relaxing absolutely helps. In fact when you get good at the clinch, you learn when you need to be tense and when you can relax. Relaxing conserves energy and makes you faster. The clinch is one of the most exhausting aspects of the fight game and learning to relax is crucial. I'd suggest pummeling at a slow, controlled pace to help build your confidence in the clinch and to learn how to relax while in it.