"You there!” (4)
'you there' 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 "Sibilant sound made to attract attention" , "Listen!" , "Surreptitious attention-seeker" , "Don't tell anyone" , "Here's a secret" .)
(Other definitions for hey that I've seen before include "Exclamation calling attention" , "Call attracting attention" , "Exclamation to call attention" .)
(Other definitions for are that I've seen before include "(We/they/you) exist" , "Is, in the plural" , "Unit of land measure equal to 100 square metres" , "Exist; square measure" , "(They/you/we) exist" .)