Heckler's word (3)
I believe the answer is:
boo
'heckler's word' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for boo that I've seen before include "Cry of disapproval as at villain" , "Shout in disgust" , "causing surprise!" , "Make one's dislike heard" , "Show displeasure orally" .)