Hissed attention-getter (4)
I believe the answer is:
psst
'hissed attention-getter' 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 "way of attracting attention" , "This is confidential" , "Sound to call quietly" , "Whispered exclamation to attract attention" , "Sound used to attract someone's attention" .)