Sheep's bleat (3)
I believe the answer is:
baa
'sheep's bleat' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for baa that I've seen before include "Call from Jacob" , "Bleat of a sheep" , "Sheep's cry" , "Sheep's bleat" , "Beastly sound f" .)