Twain protagonist (4)
I believe the answer is:
finn
'twain protagonist' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for finn that I've seen before include "Huckleberry - -" , "Someone from Helsinki?" , "adventurer in Mark Twain story" , "Nationality of a citizen of Helsinki" , "hero of American fiction" .)