Clearing choices

Let's say I've got a verbal offer from one university and have been placed in a waiting list in another university that I prefer. As it is a verbal offer, I only have 48 hours to accept this from clearing. I was wondering if it were possible to add a clearing choice of that uni while also able to change this clearing choice a few days later IF I get the offer from my preferred uni? I'd be happy with either unis but I just have preferences