Show dissatisfaction (3)
I believe the answer is:
boo
'show dissatisfaction' 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 "Poor reception" , "Frightening word (for a goose?)" , "I hated that!" , "Expression of disapproval" , "Sound expressing disapproval" .)