clench / clinch
To close tightly versus to settle or fasten.
To clench is to close tightly: clench a fist, clench your teeth. To clinch is to settle a matter, or to fasten: clinch the deal, clinch a nail. Do not use clench the deal.
Examples
Use He clenched his jaw.
Use That clinched the vote.
Avoid That clenched the vote.