Advert claiming merit so inappropriately paid off (9)
I believe the answer is:
amortised
'paid off' is the definition.
(amortising is a kind of paying off)
'advert claiming merit so inappropriately' is the wordplay.
'advert' becomes 'ad' (synonyms).
'claiming' means one lot of letters goes inside another.
'inappropriately' is an anagram indicator (I've seen this in other clues).
'merit'+'so'='meritso'
'meritso' with letters rearranged gives 'mortise'.
'ad' placed around 'mortise' is 'AMORTISED'.
(Another definition for amortised that I've seen is " Written off over time".)