serrer, crisper
Clench it.
Serrez-le.
— Like Stars on Earth (2007)
Clench your wrist.
Serrez le poignet.
— Andhadhun (2018)
Please clench your teeth.
Veuillez serrer les dents.
Then you clench your fists like this.
Ensuite, vous serrez les poings comme ceci.
— About Time (2013)