Coming back with "Nonsense, you chump, he's a clergyman!" (6)
I believe the answer is:
pastor
'clergyman' is the definition.
(pastor is a kind of clergyman)
'coming back with nonsense you chump' is the wordplay.
'coming back' says the letters should be written in reverse.
'with' is a charade indicator (letters next to each other).
'nonsense' becomes 'rot' (I've seen this before).
'you chump' becomes 'sap' (I can't justify this - if you can you should give a lot more credence to this answer).
'rot' next to 'sap' is 'rotsap'.
'rotsap' reversed gives 'PASTOR'.
'he's a' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for pastor that I've seen before include "Minister; shepherd" , "To spar with clergyman" , "Church minister" , "Padre perhaps" , "Minister of religion" .)