"Hey you!" (4)
I believe the answer is:
psst
'hey you' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for psst that I've seen before include "Sibilant interjection to call attention" , "attempt to attract attention" , "Soft sound to get someone's attention" , "Noise used to attract someone's attention privately" , "Sound used to attract someone's attention" .)