"Hey! Listen up!" (4)
I believe the answer is:
psst
'hey listen up' 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" , "Sibilant whisper to attract attention" , "Sibilant sound made to attract attention" , "Sound of call to hear a secret" , "Here's a secret" .)