Credit the fool with being stupid (5)
I believe the answer is:
crass
'being stupid' is the definition.
(thesaurus)
'credit the fool' is the wordplay.
'credit' becomes 'cr'.
'the fool' becomes 'ass' (ass is a kind of fool).
'cr'+'ass'='CRASS'
'with' is the link.
(Other definitions for crass that I've seen before include "Tactless, boorish" , "Grossly stupid, tactless" , "Grossly insensitive" , "'Gross, vulgar (5)'" , "Insensitively stupid" .)