At first Sirius is reluctant. He doesn't want to be found, after all. Doesn't want people dragging him back to prison. But at the same time, Clara's a ghost. And she's helped him before. Maybe it would help to talk about it.

"That might help, actually. But, I'm going to have to ask that you not tell anyone. Right now, the less people that know the better."

At least, until he can get some evidence or something.