Stupid fool going after credit (5)
I believe the answer is:
crass
'stupid' is the definition.
(I've seen this before)
'fool going after credit' is the wordplay.
'fool' becomes 'ass' (both can mean a stupid person).
'going after' means one lot of letters go next to another.
'credit' becomes 'cr' (abbreviation).
'ass' after 'cr' is 'CRASS'.
(Other definitions for crass that I've seen before include "Insensitively stupid" , "Indelicate" , "Asinine" , "Totally lacking in sensibility" , "Grossly insensitive" .)