Take a look at the sentence again:
If you offer Valerie a piece of watermelon, she will refuse it, the slices
look like the wicked smile of a circus clown who
once scared her as a child during a trip to the big top.
The blank space between clown and who is completely correct. The slices look like the wicked smile of a circus clown is a main clause. Who once scared her begins a subordinate clause. When a subordinate clause follows a main clause, as in the example above, you generally do not use punctuation.
Go back to the sentence to try again.