Scare word (3)
I believe the answer is:
'scare word' 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 "Exclamation to startle" , "Shout of disapproval, maybe at ref" , "Negative reaction" , "Expression of disapproval" , "Cry of disapproval as at villain" .)