Ghost's shout (3)
I believe the answer is:
boo
'ghost's shout' 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 "Frightening word (for a goose?)" , "I'm unimpressed" , "Expression of disapproval" , "Show displeasure orally" , "Disapproving jeer" .)