Mark this American author (5)
I believe the answer is:
twain
This clue is a double definition.
'mark' is the first definition.
(Mark Twain)
'this american author' is the second definition.
(Mark Twain)
(Other definitions for twain that I've seen before include "Mark, say" , "Two, formerly" , "Mark -, pen name of S. L. Clemens" , "US author Mark, attributed with the quote "golf is a good walk spoiled"" , "Two items of the same kind and well-known author" .)