Flock sound (3)
I believe the answer is:
baa
'flock sound' 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 "Statement from those flocking" , "How one of the fold may sound" , "Sound of sheep" , "Bleat of a sheep" , "Sound of a lamb" .)