[nodding!] The only way I can see it being Clover is if she didn't know Lucy would get caught by the trap and it was an accident. They're also both really sure that neither of them left that evening, and it's not like in last week's case where one of them might have been asleep. If the killer did drop the knife in the pool between 7:30 and 10:00 and it's not some sort of trick, then they were both awake during that time.