Audibly milk sheep (4)
I believe the answer is:
ewes
'sheep' is the definition.
(a ewe is a female sheep)
'audibly milk' is the wordplay.
'audibly' shows a homophone (sound like).
'milk' becomes 'use' (both can mean to exploit).
'use' sounds like 'EWES'.
(Other definitions for ewes that I've seen before include "She-sheep" , "Adult sheep" , "Sheep (females)" , "Females in the flock" , "farm animals" .)