It may make you jump (3)
I believe the answer is:
boo
'it may make you jump' 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 "get off!" , "Make one's dislike heard" , "Shout in disgust" , "A shocking exclamation!" , "Cry of disapproval as at villain" .)