After taking a deep breath, he slowly unclenched his fist.

unclenched 是动词 unclench 的过去式。unclench 由前缀 un-（表示"相反、取消"）和动词 clench（紧握、咬紧）组成，字面意思是"松开紧握"，指"放开紧握的拳头或咬紧的牙齿"。