Is it a square or round section clip? The stock part is a round section spring clip, that is slightly oversized. When inserted, it compresses down on the chamfer and slides into the splines, then when fully home, just pops out the other side, expanding again. This is what retains the flange in place in use. Removal is normally as Craig et al have said, apply some outward force, the clip compresses back and allows the flange to slide out. They can feel quite tight sometimes.

If it is a square section, then yes indeed, it’ll need removing first, but it would be non standard if it was.