Mark two (5)
I believe the answer is:
twain
I believe this clue is a double definition.
'mark' is the first definition.
(Mark Twain)
'two' is the second definition.
(archaic term for 'two')
(Other definitions for twain that I've seen before include "two in history" , "Mark -, pen name of S. L. Clemens" , "Mark, say" , "that was a low number" , "Old pair beginning" .)