Possible Clues:

• US author Mark, attributed with the quote 'golf is a good walk spoiled'
• Mark - - , US writer
• One mark or two
• I want (anag) - US novelist and humorist
• 'What Is Man?' essayist
• Mark ---, US author
• Mark ..... created Tom Sawyer
• Two items of the same kind and well-known author
• Mark how this can make one of two
• The Finn man