Berlin quartet worried about possible source of mutton roast (10)
I believe the answer is:
vituperate
'roast' is the definition.
(both can mean to criticise someone)
'berlin quartet worried about possible source of mutton' is the wordplay.
'berlin quartet' becomes 'vier' ('four' in Germany).
'worried' becomes 'ate' (something worrying may eat someone up).
'about' means one lot of letters goes inside another.
'possible source of mutton' becomes 'tup' (tup can mean a ram).
'vier'+'ate'='vierate'
'vierate' enclosing 'tup' is 'VITUPERATE'.
(Other definitions for vituperate that I've seen before include "Revile" , "Attack with abusive criticism" , "curse" , "Abuse" , "blackguard" .)