“Banter”, as in “nonsense” (5)
I believe the answer is:
roast
'banter' is the definition.
(thesaurus)
'as in nonsense' is the wordplay.
'in' indicates putting letters inside.
'nonsense' becomes 'rot' (I've seen this before).
'as' going into 'rot' is 'ROAST'.
(Other definitions for roast that I've seen before include "dish of lamb, say" , "Large piece of oven-cooked meat" , "Tear to pieces" , "Cook (meat) in oven" , "Cooked meat; criticise" .)