These words are often confused.
To clench is to hold tightly or grasp firmly.
- The boy clenched his fists in anger.
- The patient clenched the arms of his wheel chair.
Common expressions with clench are: clench one’s teeth, clench one’s fist, clench onto a rope etc.
To clinch is to win, secure or obtain.
- The player clinched the trophy with a fantastic performance.
- He clinched the deal with a handshake.