Bleat (3)
I believe the answer is:
baa
'bleat' is the definition.
(synonyms)
This is all the clue.
(Other definitions for baa that I've seen before include "Sound from a ewe" , "not quite consistent set of good A-level results?" , "Cry of a sheep" , "Beastly sound f" , "Cry of lamb, sounds angry" .)