"Hey ... over here!" (4)
I believe the answer is:
'hey over here' 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 "Hissing noise to attract attention" , "Sound used to attract someone's attention" , "Attention-seeking noise" , "way of attracting attention" , "Notice me" .)