Succeed with Italian -- you must be joking! (4,3,2)
I believe the answer is:
come off it
'you must be joking' is the definition.
(both sayings can be used to express disbelief)
'succeed with italian' is the wordplay.
'succeed' becomes 'comeoff' (I've seen this before).
'with' is a charade indicator (letters next to each other).
'italian' becomes 'it' (abbreviation for Italian).
'comeoff'+'it'='COME OFF IT!'
(Other definitions for come off it that I've seen before include "Drug addict should" , "People climbing dangerous structure may be told to" , "Stop messing about!" , "get down from there" , "You're mad!" .)