Attention-getting sound (4)
I believe the answer is:
psst
'attention-getting sound' 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 "Interjection to draw attention" , "Here's a secret" , "Don't tell anyone" , "Sound of call to hear a secret" , "This is confidential" .)