Hamlet, terrific keeping in character (6)

I believe the answer is:

letter

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

'hamlet terrific keeping in' is the wordplay.
'in' indicates a hidden word.
'LETTER' is hidden within 'hamlet terrific keeping'.

(Other definitions for letter that I've seen before include "Alphabetic symbol" , "Written message" , "A written form of communication" , "Character - landlord - epistle" , "Unit of alphabet" .)

I've seen this clue in The Telegraph.

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