As far as I remember some of the knockoffs have screw-in fixtures, some are kind of press-fit like the original x5.
So if unscrewing doesn't work try to fill the light with water (batteries removed) screw the tailcap back on and put it in the freezer. The fuxture should come out a little when the water turns to ice and expands. You might have to reapeat this some times untill it comes off. Others said it works with the x5, it might work with your clone too.
Chrisse