Mean rebuke over fellow with evidence of debts (12)
I believe the answer is:
parsimonious
'mean' is the definition.
(I've seen this before)
'rebuke over fellow with evidence of debts' is the wordplay.
'rebuke' becomes 'rap' (both can mean to criticise or reprimand).
'over' says the letters should be written in reverse.
'fellow' becomes 'simon' (I've seen this before).
'with' means one lot of letters go next to another.
'evidence of debts' becomes 'IOUs' (IOU's or I Owe You's).
'rap' back-to-front is 'par'.
'par'+'simon'+'ious'='PARSIMONIOUS'
(Other definitions for parsimonious that I've seen before include "Thrifty, mean" , "Frugal" , "Penny-pinching" , "Tight-fisted" , "Miserly" .)