Here is 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 comma between it and the is illegally connecting two main clauses. She will refuse it is the first main clause. The slices look like the wicked smile ... begins the second main clause. Main clauses can stand alone as complete sentences, so they require a stronger break between them than a wimpy comma.
To fix this problem, you could put a period after it and capitalize the T for the.