After the tension passed, he slowly 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"。unclenched 在这里描述从紧握状态变为松开的状态。