Obsolete character to abandon? Spy may have a use for it (4-6,4)

I believe the answer is:

dead-letter drop

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'spy may have a use for it' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.

'obsolete character to abandon?' is the wordplay.
'obsolete' becomes 'dead' (similar in meaning).
'character' becomes 'letter' (synonyms).
'to abandon?' becomes 'drop' (I've seen this before).
'dead'+'letter'+'drop'='DEAD-LETTER DROP'

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I've seen this clue in The Times.

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