Tactless idiot, after credit (5)
I believe the answer is:
crass
'tactless' is the definition.
(I've seen this before)
'idiot after credit' is the wordplay.
'idiot' becomes 'ass' (I've seen this before).
'after' says to put letters next to each other.
'credit' becomes 'cr'.
'ass' after 'cr' is 'CRASS'.
(Other definitions for crass that I've seen before include "Blundering" , "Coarse" , "'Gross, vulgar (5)'" , "Grossly stupid, tactless" , "Insensitively stupid" .)