Crossword Genius

Shout after sneaking up on someone (3)

Ross

I believe the answer is:

boo

(Other definitions for boo that I've seen before include "Sound expressing disapproval" , "Poor reception" , "Disapproving sound" , "A shocking exclamation!" , "Frightening word (for a goose?)" .)

I've seen this clue in The New York Times.
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