Somewhat athletic Len chooses grip (6)
I believe the answer is:
clench
'grip' is the definition.
(I know that grip can be written as clench)
'somewhat athletic len chooses' is the wordplay.
'somewhat' indicates the answer is hidden within the clue.
'CLENCH' is hidden in the letters of 'athletic len chooses'.
(Other definitions for clench that I've seen before include "Sharply contract (muscle)" , "Close, grasp, tightly" , "Close teeth or fingers tightly" , "Grip tightly" , "Close firmly together" .)