A clone is a copy of another AEG company’s design, which was developed through their own research and development.
Most clones are cheaper because they don't have to spend the capital on R&D. In the beginning all clones, copied TM versions. Everything has some type of TM development in them. Then over time some clone companies started to make their own improvements to the overall design. Gearbox wise, everything is basically a TM design. What can vary is materials/parts used, gun take down, furniture and overall gun quality.
I have noticed that some clones have cheap or poorly made outter parts, which make the gun feel more fake than it all ready is. While others are made very well, but that is just me.