"Want to hear a secret?" (4)
I believe the answer is:
'want to hear a secret?' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for psst that I've seen before include "Quiet call for attention" , "Whispered attention grabber" , "Beckoning sound" , "Soft sound to get someone's attention" , "(Whispered) may I have your attention?" .)