A respectable, short debtor’s note ultimately proves damaging (10)
I believe the answer is:
pernicious
'damaging' is the definition.
(thesaurus)
'a respectable short debtor's note ultimately proves' is the wordplay.
'a' becomes 'per' (eg 'one a day' means 'one per day').
'respectable' becomes 'nice' (similar in meaning).
'short' means to remove the last letter.
'debtor's note' becomes 'IOU' (note of a debtor to be paid).
'ultimately' suggests the final letters.
The final letter of 'proves' is 's'.
'nice' with its last letter removed is 'nic'.
'per'+'nic'+'iou'+'s'='PERNICIOUS'
(Another definition for pernicious that I've seen is " Destructive and harmful".)