Sibilant summons (4)
I believe the answer is:
psst
'sibilant summons' 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 "(Whispered) may I have your attention?" , "Sound used to attract someone's attention" , "Noise used to attract someone's attention privately" , "Call to attract attention" , "Sibilant sound to get someone's attention" .)