Review the item:
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 an adjective clause.
When an adjective clause follows a nonspecific noun like clown, you do not use punctuation.