Call to attention? (4)
I believe the answer is:
psst
'call to attention?' 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 "Call to attract attention" , "Sibilant sound made to attract attention" , "Sibilant interjection to call attention" , "Interjection to draw attention" , "Quiet call for attention" .)