Background: On the way to work, a radio host was caught in a fight with a sinister girl trying to rip his throat out. He drove away from her back to the station, but what made it even worse was that the girl, who didn't recognize the host was the guy she'd just attacked, called into his show and told him what had just happened. Now he was more on edge, fearful that she'd call back and expose his heinous crime.He opened up the phone lines again for requests. He kept his finger on the drop button fearful that he was going to get another call from the girl. Only this time she was going to call him out and publicly accuse him of trying to rape and murder her.
Every time he heard a female voice he clinched up a little bit.
Source: Public Enemy Zero ,Andrew Mayne
I couldn't find a good definition that would fit the context, so I assume it's similar to "clench up". Does "clinched up a little bit" mean clenching his fists, his teeth or tightening up his muscles?