Attention-getting whisper
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 "attempt to attract attention" , "Whispered attention grabber" , "Pay attention" , "Don't tell anyone" , "Whispered exclamation to attract attention" .)