Dimwit’s stupefying drug (4)
I believe the answer is:
dope
This clue is a double definition.
'dimwit's' is the first definition.
(both can mean a stupid person)
'stupefying drug' is the second definition.
(dope can mean cannabis)
(Other definitions for dope that I've seen before include "Drugs for a dimwit" , "But does he understand" , "Simpleton" , "Information; nitwit" , "Informal drug or stupid person" .)