What Is A Clean Room Implementation?

Bryan Brunton asks: "I am writing an open source game called Merchant Empires For now, it is almost completely based on a closed source game called Space Merchant. IMO, Space Merchant is slow, ugly, and bug-filled. I am getting Space Merchant users who log into my game, spew some profanity and claim that I will be sued. I have no access to Space Merchant source code. Can I be sued? If so, what do I need to do to make my game qualify as a clean room implementation?"