Endorse a letter (7)

I believe the answer is:

initial

This clue is a double definition.

'endorse' is the first definition.
(I've seen this before)

'a letter' is the second definition.
(initial is a kind of letter)

(Other definitions for initial that I've seen before include "Introductory" , "subscribe, minimally" , "First letter (of a word)" , "write two or three letters, usually" , "Sign" .)

I've seen this clue in the Evening Standard.

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