Ghost's cry (3)
I believe the answer is:
boo
'ghost's cry' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for boo that I've seen before include "Disapproving sound" , "A shocking exclamation!" , "Cry of contempt" , "Shout of disapproval, maybe at ref" , "I hated that!" .)