The communication granting permission? (6)

I believe the answer is:

letter

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'the communication' is the definition.
(letter is a kind of communication)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for letter that I've seen before include "An alphabetic unit" , "Written message" , "Written character - landlord" , "Character - landlord - epistle" , "Character; missive" .)

I've seen this clue in the Evening Standard.

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