its the top joint in the hub in this pic right?
the ball joint has a taper which fits into a taper on the hub like so(58 seats into 60):
you need to whack the balljoint to make the taper lock up in the hub. so dont hit the threaded end.
The circlip prbably holds the ball joint into the strut, well sorta as its a press fit. you cant take the ball joint out of the strut until its removed from the hub tho.