Blundering idiot on credit! (5)
I believe the answer is:
crass
'blundering' is the definition.
(I've seen this before)
'idiot on credit' is the wordplay.
'idiot' becomes 'ass' (I've seen this before**).
'on' says to put letters next to each other.
'credit' becomes 'cr'.
'ass' after 'cr' is 'CRASS'.
(Other definitions for crass that I've seen before include "Unrefined" , "Showing gross lack of sensitivity" , "'Gross, vulgar (5)'" , "Indelicate" , "Insensitively stupid" .)