"It used to be that you could sign onto the carnival for a year and a day in exchange for something." Before everything went to shit, that is. "I signed on to learn about a particular spell. I didn't know how it was cast, I wanted to learn, and now I do."

He's being very circumspect, mostly because that particular spell is technically necromancy. If Beau wants to know the answer, he'll definitely tell her: there's something about this woman that Strange likes and he's enjoyed all his conversations with her so far. But again, necromancy. You don't drop that information in casual conversation.