On the radio employ sheep (4)
I believe the answer is:
ewes
'sheep' is the definition.
(I've seen this before)
'on the radio employ' is the wordplay.
'on the radio' indicates a 'sounds like' (homophone) clue.
'employ' becomes 'use' ('use' can be a synonym of 'employ').
'use' sounds like 'EWES'.
(Other definitions for ewes that I've seen before include "Female sheep" , "members of flock" , "farm animals" , "wool producers" , "Sheep (females)" .)