Expression of disapproval (3)
I believe the answer is:
boo
'expression of disapproval' is the definition.
(as in a crowd booing)
This is the entire clue.
(Other definitions for boo that I've seen before include "Sound made to show disapproval or contempt" , "Surprising outburst" , "Poor reception" , "I disapprove" , "Expression of disapproval" .)