Bossy's sound (3)
I believe the answer is:
moo
'bossy's sound' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for moo that I've seen before include "Farmyard sound" , "Cow's low" , "Bovine low" , "Cartoon sound of a cow" , "Cow's call" .)