Horrible old chest holding new debtor's notes (9)
I believe the answer is:
obnoxious
'horrible' is the definition.
(thesaurus)
'old chest holding new debtor's notes' is the wordplay.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'chest' becomes 'box' (both can mean a square container).
'holding' is an insertion indicator.
'new' becomes 'n' (abbreviation as in New Testament NT).
'debtor's notes' becomes 'ious' (I've seen this in another clue).
'box' enclosing 'n' is 'bnox'.
'o'+'bnox'+'ious'='OBNOXIOUS'
(Other definitions for obnoxious that I've seen before include "Loathsome" , "Extremely unpleasant" , "Repugnant" , "Nauseating" , "Foul" .)