"Clone" refers to a gun that is a rebranding of another. Essentially, one company designs a gun and manufactures it. Then another company takes the design and tweaks it slightly, buys the rights, or somehow replicates the gun. This gun is then produced using (generally) lower quality materials. This makes clones cheaper than the original gun, though (again, generally) lower quality. Well known clone companies include Echo 1, UTG, and many others.