I am not afraid of clowns. However, I have a (non autistic) nephew (7 years old) who is.
About two years ago my sister and I took him to the Indiana state fair. What we didn't realise is that it was clown day!
Anyway, to make a long story short I spent several moments shielding (distracting him, getting him focused on some other thing when a clown came in to range) him from clowns. He also has a fear of Santa Claus as well. He loves Santa but will not sit on his lap or be in close proximity of him. In other words 'I love you Santa, but stay away from me!' is his mantra.
I suspect that he will overcome these fears as he gets older.