Devitalize (3)
I believe the answer is:
sap
'devitalize' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sap that I've seen before include "Undermine the tree juice" , "Imbecile" , "Fluid in plant stem" , "Vital plant juice" , "One is easily fooled" .)