v. 25B6; verb
he stood there clenching his hands : SQUEEZE TOGETHER, clamp together, close/shut tightly; make into a fist.
he clenched the back of chair : GRIP, grasp, grab, clutch, clasp, hold tightly, seize, press, squeeze.
25B6; noun the clench of his fists : CONTRACTION, tightening, tensing, constricting.