Partake in exercise, fool! (4)
I believe the answer is:
dope
'fool' is the definition.
(I've seen this before)
'partake in exercise' is the wordplay.
'partake in' becomes 'do' (to partake in something is to do it).
'exercise' becomes 'pe' (Physical Education).
'do'+'pe'='DOPE'
(Other definitions for dope that I've seen before include "The idiot's a drug" , "Weed perhaps" , "Fool - model aeroplane varnish" , "Informally, drug or silly person" , "Lowdown; drugs (informal)" .)