Apply (force) (5)
I believe the answer is:
exert
'apply force' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for exert that I've seen before include "Cause (oneself) to make a strenuous effort" , "Apply (pressure)" , "Make a great effort" , "Bring quality or influence into play" , "Put to use, vigorously" .)