Stupid fool's after credit (5)
I believe the answer is:
crass
'stupid' is the definition.
(thesaurus)
'fool's after credit' is the wordplay.
'fool's' becomes 'ass' (ass is a kind of fool**).
'after' is a charade indicator (letters next to each other).
'credit' becomes 'cr'.
'ass' after 'cr' is 'CRASS'.
(Other definitions for crass that I've seen before include "Boorish about scars" , "Grossly insensitive" , "Indelicate" , "Totally lacking in sensibility" , "Insensitively stupid" .)