"Pipe down!"
(Another definition for shush that I've seen is " Interjection for keep quiet, don't make noise".)
'pipe down' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for hush that I've seen before include "Make (someone) quiet" , "Sudden descent of silence" , "Soothe" , "Noiselessness" , "(Call for) quiet" .)