Found this on Twitter, and this is actually quite a cracktastic gif. Spoilers for companion's alleged name and costumes/era for the Christmas special XD
GET IT?? Back to the Future III, where Doc meets a girl named Clara in the 19th century.
And now we apparently have the Doctor ... meeting a girl named Clara ... in the 19th century XD
Moffat either gets points for the most random tribute EVER, or for her name not really being Clara and this being a total swerve that only a few Children of the 80s will pick up on XD