Complete write off (5)
I believe the answer is:
total
I believe this clue is a double definition.
'complete' is the first definition.
(similar in meaning)
'write off' is the second definition.
(both can mean to permanently damage something)
(Other definitions for total that I've seen before include "The whole, sum" , "Whole amount" , "Sum, aggregation" , "Aggregate, sum" , "Entire sum" .)