Zapple is certainly a made up word. Two things to think about though. The first is that the word zapple is just apple with a Z added onto it. So the cart probably contains something similar to apples, and looks like an apple cart. The second thing is, why didn't the writer just use the word apple? Perhaps the apples were changed somehow. Maybe they were cooked, or fried. Strange food is often sold at the circus. If the book is about a fantasy world, the author might be making up names of fantasy fruit. Similar to real fruit, but not the same. It's also possible that the author meant to type "apple", and hit the Z key by mistake. Typing errors do happen.

I think it is a made-up word from a book. I've never heard it before. A cart is some kind of wheeled vehicle like a donkey cart or a go-cart. The author is using the term "zapple cart" to refer to some kind of vehicle at the circus.