"Clever boy," she says with a dimpled grin at him, and then she takes wards. "Meet you back in the middle, Doctor. Off you pop."

With that, Clara takes off with her wards, setting them in an arc that will connect to his, effectively sealing in the dock. She's angry about everything that's happened, about the people who've made it happen all by hurting a child. If this doesn't work, at least she can say she stands by her decision to try.