Credit a mug with being stupid (5)
I believe the answer is:
crass
'being stupid' is the definition.
(thesaurus)
'credit a mug' is the wordplay.
'credit' becomes 'cr'.
'a mug' becomes 'ass' (I've seen this before).
'cr'+'ass'='CRASS'
'with' is the link.
(Other definitions for crass that I've seen before include "Tactless, boorish" , "Unrefined" , "Showing gross lack of sensitivity" , "Coarse" , "Indelicate" .)