Attention-getting whisper (4)
I believe the answer is:
psst
'attention-getting whisper' 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 "Sound used to attract someone's attention" , "Whispered exclamation to attract attention" , "This is confidential" , "(Whispered) may I have your attention?" , "Listen!" .)