Sign missive (6)

I believe the answer is:

letter

I believe this clue is a double definition.

'sign' is the first definition.
(letters are symbols or signs)

'missive' is the second definition.
(I know that missive can be written as letter)

(Other definitions for letter that I've seen before include "Unit of alphabet" , "Written character - landlord" , "Item sent - character" , "Epistle, missive" , "Character; missive" .)

I've seen this clue in The Telegraph.

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