From King Dictionary of Contemporary Englishclenchclench /klentʃ/ ●○○ verb [transitive] 1 clench your fists/teeth/jaw etc2 to hold something tightly in your hand or between your teeth a cigar clenched between his teeth→ See Verb table
Examples from the Corpus
clenchMy apron flapped, my teeth were clenched.He succeeded in pulling apart my clenched arms and started on my legs.His buttocks were clenched as he moved towards her.She clenched her fists, on the verge of tears.The shock was terrific, he gasped into his oxygen mask, his hands clenching involuntarily.I clenched out the light and stuck my fingers in my ears.Her muscles involuntarily clenched, then went limp.
Origin clench Old English beclencan