ATMachine said:

Hmmm..... who is "The Witch's Familiar," I wonder?

The "Witch" in question is presumably Missy.... but who's the Clara to her Doctor?

I wonder if they're setting up a new companion for when Jenna finally leaves the show. Here's hoping they don't go with somebody else from present-day Earth this time.

OR it could be the witch is the one from the Night Of The Doctor, since it states that the actress is returning as well. Maybe the episode could explain possibly how Missy came about.

Then again, your idea makes more sense in terms of what the title seems to suggest.