Possible Clues:

• US author; pair (arch.)
• Old pair beginning to tinker with cart
• Tom Sawyer author
• Wit who wrote 'When in doubt, tell the truth'
• Mark . . . . . created 'Huckleberry Finn'
• American writer Tennessee absorbing Washington Irving's lead
• Sibling conceals a novelist
• Mark, of Finn fame
• Author depicted next to a steamboat on a 2011 stamp
• Mark in American literature, and two in history