Discouraging word (3)
I believe the answer is:
boo
'discouraging word' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for boo that I've seen before include "Disapproving jeer" , "Startling utterance" , "get off!" , "Express noisy disapproval" , "Cry of contempt" .)