Write off sum (5)
I believe the answer is:
total
This clue is a double definition.
'write off' is the first definition.
(both can mean to permanently damage something)
'sum' is the second definition.
(I know that sum can be written as total)
(Other definitions for total that I've seen before include "French oil and gas company" , "Complete, entire amount" , "The whole number" , "Aggregate amount" , "energy provider" .)