My clown does not really bite...he hits...like when he is going after a feeder, except he has no intention of eating the thing he is hitting. He just leaves huge 2-3 inch circles on the side of my oscar.
Personally I'd be worried about the endli because it might just be a long noodle to the clown if the endli is not big enough.
a not about the clown knifes bite is that they don't really have sharp cutting teeth like the aba aba, they actually have many small teeth intended for holding prey, and they don't have a lot of biting power like the aba aba. So its possible for the ck to scuff up your endy.
Depending on how you introduce them to each other,since they are both pretty mellow predators they may accept one another with no problems