Possible Clues:

• With 33-Down, co-author of "The Communist Manifesto"
• Mathematician Weierstrass dubbed the 'father of modern analysis'
• __ 21 down, author of Capital
• Philosopher Marx
• Urban of "The Boys"
• Marxism's Marx
• 'Das Kapital' author Marx
• Name that's an anagram of 'lark'
• Political thinker Marx
• Two-time NBA MVP Malone known as 'The Mailman'