When a child says, "you're not a real clown", you can take it with anger and defend yourself or you can think of what's going on developmentally and have a ready SIMPLE come-back.
Children are transitioning from a magical world to a world of reality. When you are 3, 4 or 5 years old, your parents are super heros (they move super fast, they are super tall, can control super hot things like a stove, have super strength, they can lift me with one hand) to the reality we all live in (the stars in the sky are millions of stars billions of miles away, beds need to be made, food does not magically appear on the table, it needs to be paid for).
Children will ask questions or make statements only as far as they want to go. Wondering about the reality of whether you're a clown is in the same realm of "where do babies come from?"
Children are testing the water of what's magic and what's real. So, is a clown real? Clowns have a wonderful warped logic. That means a clown has no control because things tend to get messed up and absolute control of all situations, because to bungle things you have to know an awful lot about your own character. My simple response is to say, "you've found me out" tip my hat, as I put my hat back on my head it falls off. I start with control, the agreement, the universal tip of the hat, the unexpected hat falling to the floor, the loss of control.
My giving a simple nod of the hat which falls off, answers the question, "Of course I'm a real clown." I give them a few more years of magic in their lives. They never have a follow up question after. If they did, I tip my hat again and watch it fall to the floor in anguish.