Listen, lightweight — you shut up! (4)
I believe the answer is:
oyez
'listen' is the definition.
(term used by town criers to get attention)
'lightweight you shut up' is the wordplay.
'lightweight' becomes 'oz' (I've seen this in other clues).
'you' becomes 'ye'.
'shut up' means one lot of letters goes inside another (inserted letters get shut in or shut up).
'oz' placed around 'ye' is 'OYEZ'.
(Other definitions for oyez that I've seen before include "Call of hte old Town Crier" , "Town crier's utterance" , "Town crier's 'Attention!'" , "Town crier's shout" , "Town crier's call (Z, not S)" .)