Be quick to punish taking cocaine (6)
I believe the answer is:
hasten
'be quick' is the definition.
(I've seen this before)
'punish taking cocaine' is the wordplay.
'punish' becomes 'chasten' (chasten could historically mean to punish).
'taking' indicates named letters should be taken away.
'cocaine' becomes 'c' (drugs abbreviation).
'chasten' with 'c' taken away is 'HASTEN'.
'to' acts as a link.
(Other definitions for hasten that I've seen before include "Step up" , "Speed up" , "Make faster" , "Move quickly" , "Athens (anag.)" .)