Say, Ron's brother's admitting evidence of debt? That's appalling! (9)
I believe the answer is:
egregious
'that's appalling' is the definition.
(I've seen this before)
'say ron's brother's admitting evidence of debt?' is the wordplay.
'say' becomes 'eg' (both can mean 'for example').
'ron's brother' becomes 'gre' (I can't explain this - if you can you should believe this answer much more).
'admitting' is an insertion indicator.
'evidence of debt?' becomes 'IOUs' (IOU's or I Owe You's).
'eg' enclosing 'gre' is 'egreg'.
'egreg'+'ious'='EGREGIOUS'
Can you help me to learn more?
(Other definitions for egregious that I've seen before include "Notably bad" , "Flagrant; appalling" , "Outstandingly villainous" , "Shocking" , "Outstandingly bad" .)