From Longman Dictionary of Contemporary Englishclench your fists/teeth/jaw etcclench your fists/teeth/jaw etcHOLDto hold your hands, teeth etc together tightly, usually because you feel angry or determined Jody was pacing the sidelines, her fists clenched. → clench
Examples from the Corpus
clench your fists/teeth/jaw etc• Papa clenched his fists and lips in the dark wood.• He clenched his fists and remained where he was.• He clenched his fists even tighter.• He clenched his fists in frustration and annoyance.• He clenched his teeth, pulled back his shoulders and began to stride up the road.• He clenched his teeth together but the first syllable forced itself around the corner of his mouth.• I clenched my teeth, wondering what to do now. 