Permissive missive? (6)

I believe the answer is:

letter

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

I can't explain the remainder of the clue.

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(Other definitions for letter that I've seen before include "Unit of alphabet" , "who may offer terms to residents?" , "A, B or C, for example" , "It's one of twenty-six" , "An alphabetic unit" .)

I've seen this clue in the Evening Standard.

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