Answer: clenching

AnswerCrossword Clue
clenching(of a muscular part of the body) Tighten or contract sharply, esp. with strong emotion
clenching(with reference to the teeth) Press or be pressed tightly together, esp. with anger or determination or so as to suppress a strong emotion
clenchingGrasp (something) tightly, esp. with the hands or between the teeth
clenchingCLENCH, to grasp firmly
clenching(with reference to the fingers or hand) Close into a tight ball, esp. when feeling extreme anger