Drudge to take exercise on (4)
I believe the answer is:
peon
'drudge' is the definition.
('peon' can be a synonym of 'drudge')
'exercise on' is the wordplay.
'exercise' becomes 'pe' (Physical Education).
'pe'+'on'='PEON'
'to take' is the link.
(Other definitions for peon that I've seen before include "Latin American farm labourer" , "Spanish-American farm labourer" , "lowly farm worker" , "Spanish-American farm worker" , "Spanish-American menial worker" .)