Sibilant attention getter (4)
I believe the answer is:
psst
'sibilant attention getter' 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" , "attempt to attract attention" , "Sound to call quietly" , "Quiet attention-seeker" , "Interjection to attract someone's attention without alerting others" .)