Being an extremely rare golem magician would, no doubt, mean no execution for Clarisse, but it's also her one and only secret. That being said, as long as the duke is in no such danger that Clarisse would need to openly help him, it's certainly the correct choice to remain silent about it. There's still a lot of time until her execution. I reckon the king absolutely would not leave a great power like a golem magician with the duke if the cat was let out of the bag.