Pseudonym of Samuel Langhorne Clemens (4,5)

I believe the answer is:

mark twain

(Other definitions for mark twain that I've seen before include "Tom had him writing about?" , "Writer of Huckleberry Finn" , "US writer" , ""The Adventures of Huckleberry Finn" author" , "American humorous writer, d. 1910" .)

I've seen this clue in The Guardian.

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