Sneaky "Hey!" (4)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
psst
(Other definitions for psst that I've seen before include "It's said to draw attention" , "Quiet call for attention" , "Quiet attention-seeker" , "Sibilant sound to get someone's attention" , "Sibilant sound to attract attention furtively" .)