Enfeeble (6)
I believe the answer is:
weaken
'enfeeble' is the definition.
(I know that enfeeble is a more specific form of the action weaken)
This is all the clue.
(Other definitions for weaken that I've seen before include "flag" , "Become less strong" , "Lessen the strength of" , "Lose force" , "Reduce in strength" .)