Today we have one of those pairs that are often confused. It makes my teeth clench every time I see them used improperly.
Clinch is to fix or secure (as a nail or bolt) by bending down or flattening the end that protrudes. This definition gave rise to the now more common use of clinch—to settle definitely and conclusively. It’s now a very common sports term when a team or person secures a title or victory.
Clench means to close tightly, or to grasp or grip tightly. It’s also a noun referring to a tight grip or grasp.
This is also a great article comparing the two: http://depressioncookies.blogspot.com/2015/01/clench-vs-clinch-grammar-writing-tips.html