Exercise, we hear, for sheep (4)
I believe the answer is:
ewes
'sheep' is the definition.
(I've seen this before)
'exercise we hear' is the wordplay.
'exercise' becomes 'use' ('use' can be a synonym of 'exercise').
'we hear' shows a homophone (sound like).
'use' is a homophone of 'EWES'.
'for' is the link.
(Other definitions for ewes that I've seen before include "Adult sheep" , "farm animals" , "Females in the flock" , "wool producers" , "She-sheep" .)