He took a deep breath and unclenched his fists.

unclenched 是动词 unclench 的过去式。unclench 由前缀 un-（表示"相反动作"）和动词 clench（"紧握、咬紧"）组成，字面意思是"松开紧握的东西"，指"松开拳头"或"放松紧咬的牙关"。