Voice disapproval (3)
I believe the answer is:
boo
'voice disapproval' is the definition.
(to boo is to express disapproval)
This is the entire clue.
(Other definitions for boo that I've seen before include "Startling utterance" , "Frightening word (for a goose?)" , "I'm unimpressed" , "Expression of disapproval" , "get off!" .)