Mark 2? (5)

I believe the answer is:

twain

I believe this clue is a double definition.

'mark' is the first definition.
(Mark Twain)

'2?' is the second definition.
(twain is a kind of 2)

(Other definitions for twain that I've seen before include "Mark -, pen name of S. L. Clemens" , "Two of the same kind, archaically" , "two in history" , "Old couple" , "Mark -, Huckleberry Finn author" .)

I've seen this clue in The Guardian.

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