Sent letter that's now valueless (7-3)

I believe the answer is:

written-off

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'now valueless' is the definition.
I can't tell whether this defines the answer.

'sent letter' is the wordplay.
'sent' becomes 'written' ('write' can be a synonym of 'send').
'letter' becomes 'off' (I am not sure about this - if you are sure you should believe this answer much more).
'written'+'off'='WRITTEN-OFF'

'that's' is the link.

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

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