Stupid fool is after credit (5)
I believe the answer is:
crass
'stupid' is the definition.
(thesaurus)
'fool is after credit' is the wordplay.
'fool' becomes 'ass' (ass is a kind of fool**).
'is after' means one lot of letters go next to another.
'credit' becomes 'cr'.
'ass' after 'cr' is 'CRASS'.
(Other definitions for crass that I've seen before include "Grossly insensitive" , "'Gross, vulgar (5)'" , "Tactless, boorish" , "Asinine" , "Coarse" .)