Soft "Hey, you!" (4)
I believe the answer is:
psst
'soft hey you' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for psst that I've seen before include "Sibilant sound to attract attention furtively" , "Sound to call quietly" , "Quiet attention-seeker" , "Surreptitious attention-seeker" , "Listen!" .)