Employ sound sheep (4)
I believe the answer is:
ewes
'sheep' is the definition.
(a ewe is a female sheep)
'employ sound' is the wordplay.
'employ' becomes 'use' (synonyms).
'sound' indicates a 'sounds like' (homophone) clue.
'use' is a homophone of 'EWES'.
(Other definitions for ewes that I've seen before include "She-sheep" , "members of flock" , "Sheep (females)" , "Adult sheep" , "wool producers" .)