I'm just trying to think of a quick way you can achieve this without me being able to look at your db, as my memory of my document steps has faded in the last few months. If you'd prefer someone takes a look at your database and gives you more specific advice then I would suggest raising a support case with your provider.
I am wondering if you can achieve what you want, in a slightly different way, by adding in another pre-condition? Perhaps something to check whether the Slot currently selected on the Request window is already marked as booked. If it is booked then you are not given the book action - so this precondition needs to be placed in your process before the manual book action. Perhaps you might need a label on your window that says "You will need to clear the slot field before you can book a slot".
Apart from this, if you wanted the field to clear automatically, I think we would need a bit more of a rethink of the design because the way we implemented this was to keep all values on the one Request window and then use the actions to update the CI. As it is all on the Request window, the values will remain from previous actions. If you didn't want this you'd have to look at using actions which store information in a collection object. I think it would need quite a bit of a redesign though.