Drug for a fool (4)
I believe the answer is:
dope
This clue is a double definition.
'drug' is the first definition.
(colloquial)
'for a fool' is the second definition.
(both can mean a stupid person)
(Other definitions for dope that I've seen before include "cheat thereat?" , "Drug - information" , "Booby" , "Dolt -- drug" , "Marijuana - fool" .)