One's amusing letter (9)

I believe the answer is:

character

I believe this is a double definition.

'one's amusing' is the first definition.
(character can mean an amusing person)

'letter' is the second definition.
(I've seen this before)

(Other definitions for character that I've seen before include "eccentric individual" , "Type - letter" , "Role - nature" , "case" , "Letter or figure" .)

I've seen this clue in The Telegraph.

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