Uttered to magistrate, "A new mouthpiece!" (9)
I believe the answer is:
spokesman
'new mouthpiece' is the definition.
'spokesman' can be an answer for 'mouthpiece' (spokesman is a kind of mouthpiece). I'm not sure about the 'new' bit.
'uttered to magistrate a' is the wordplay.
'uttered' becomes 'spoke' ('speak' can be a synonym of 'utter').
'to magistrate' becomes 'sm' (I can't explain this - if you can you should believe this answer much more).
'a' becomes 'an'.
'spoke'+'sm'+'an'='SPOKESMAN'
Can you help me to learn more?
(Other definitions for spokesman that I've seen before include "Voice" , "Representative at press conference" , "One speaking for others" , "One who speaks for others" , "wheelwright?" .)