Sibilant sound to get attention (4)
I believe the answer is:
psst
'sibilant sound to get attention' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for psst that I've seen before include "Whispered attention grabber" , "Sound to call quietly" , "Beckoning sound" , "Interjection to draw attention" , "attempt to attract attention" .)