Confirming clearing choice on ucas?

I called up a uni and they confirmed my place. they sent me an email on what i should to in order to proceed with accepting their offer and its quite vague:

f you would like to proceed with accepting our offer through Clearing, please enter the University of Surrey and the Politics course of your preference via UCAS track under Choice 7. We will then process your request as soon it becomes available to us. We are aiming to have all offers confirmed by 12 noon Friday 20th August.

Does this mean i should add a clearing choice and fill in the details of they gave me? Because I already got an offer and I talked to them.

Yep.

I dont get this choice 7 thing. :/

It's just how they've worded it, some weird UCAS mumbo jumbo. You just need to put it as your clearing choice and then that is you done.