Weaken fool (3)
I believe the answer is:
sap
I believe this is a double definition.
'weaken' is the first definition.
(I've seen this in another clue)
'fool' is the second definition.
(synonyms)
(Other definitions for sap that I've seen before include "Charlie" , "Enfeeble" , "Fluid in plant stem" , "trench used in siege" , "Drain of energy" .)