Take exercise drug (4)
I believe the answer is:
dope
'drug' is the definition.
(dope is a kind of drug)
'take exercise' is the wordplay.
'take' becomes 'do' ('To take' a course is 'to do' a course).
'exercise' becomes 'pe' (Physical Education).
'do'+'pe'='DOPE'
(Other definitions for dope that I've seen before include "Illegal drug; idiot" , "Information; nitwit" , "Drugs for a dimwit" , "Idiot; information" , "Dumb-bell" .)