Take exercise, a drug (4)
I believe the answer is:
dope
'a drug' is the definition.
(colloquial)
'take exercise' is the wordplay.
'take' becomes 'do' (eg taking or doing drugs).
'exercise' becomes 'pe' (Physical Education).
'do'+'pe'='DOPE'
(Other definitions for dope that I've seen before include "Informal drug or stupid person" , "Information; nitwit" , "Fool - model aeroplane varnish" , "cheat thereat?" , "Drug - information" .)