This tribute was actually better than Esse's replays (no offense meant, i know it's old, but you can do better, Esse)

I always use the term "clean" in the way that no other joint is damaged, but there's probably a majority here that means "flawless" when they say "clean".
I never understood the point of pointing out flawless kicks other than making records because I think "clean" kicks require more skill than "flawless" ones and flawless kicks don't really look better.

Please note this is just my two cents and maybe there are more mysteries between this two words than I am able to understand.