Flog heroin? Only a little wee joint (9)
I believe the answer is:
horsewhip
'flog' is the definition.
(horsewhipping is a kind of flogging)
'heroin? only a little wee joint' is the wordplay.
'heroin?' becomes 'horse' ('horse' can be a synonym of 'heroin').
'only a little' says to take the initial letters.
'joint' becomes 'hip' (hip is a kind of joint**).
The initial letter of 'wee' is 'w'.
'horse'+'w'+'hip'='HORSEWHIP'
(Other definitions for horsewhip that I've seen before include "Beat, lash" , "He worship (anag) - lash" , "punish painfully" , "thrash" , "Chastise" .)