The most common and most likely answer to this crossword clue is the 9 letter word AMORTISED
I've seen this crossword clue in The Daily Telegraph
A
M
O
R
T
I
S
E
D
Buddy explains!

The straight part
We know that "eventually wrote off" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case eventually wrote off = amortised.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does AMORTISED mean?
AMORTISED verb- To alienate (property) in mortmain.
- To wipe out (a debt, liability etc.) gradually or in installments.
- To even out the costs of running an algorithm over many iterations, so that high-cost iterations are much less frequent than low-cost iterations, which lowers the average running time.