Masterly unknown character, is he? (3)

I believe the answer is:

max

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'masterly unknown' is the definition.
The definition suggests a singular noun which matches the answer.

'character is he?' is the wordplay.
'character' becomes 'm' ().
'is he?' becomes 'Ax'.
'm'+'ax'='MAX'

Can you help me to learn more?

(Another definition for max that I've seen is " The greatest".)

I've seen this clue in the Evening Standard.

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