Shout of disapproval (3)
I believe the answer is:
boo
'shout of disapproval' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for boo that I've seen before include "Startling utterance" , "Exclamation of disapproval" , "Cry of disapproval as at villain" , "Disapproving jeer" , "causing surprise!" .)