Ghostly greeting (3)
I believe the answer is:
boo
'ghostly greeting' 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 "Make one's dislike heard" , "Shout of dislike" , "Show displeasure orally" , "Express noisy disapproval" , "Voice disapproval" .)