Flog drug, weak joint (9)
I believe the answer is:
horsewhip
'flog' is the definition.
(horsewhipping is a kind of flogging)
'drug weak joint' is the wordplay.
'drug' becomes 'horse' (horse is a slang term for heroin).
'weak' becomes 'w' (physics symbol as in W-boson).
'joint' becomes 'hip' (hip joint is a kind of joint).
'horse'+'w'+'hip'='HORSEWHIP'
(Other definitions for horsewhip that I've seen before include "flog" , "thrash" , "administer punishment" , "Coachman''s lash" , "He worship (anag) - lash" .)