Quite a bit I suspect - there's a metal collar in there that the circlip holds down. Not sure how you would open it up enough to allow the circlip free passage through it!
I would have thought that either the shock push-rod is too thin (ie - wrong shock type for the car) or the top mount collar is too large (wrong top mount).
When you put the top mount over the push-rod, there should be very little rattle room - it should be a close fit. If not, one or other of the parts must be wrong.