The BBC site had announced that directly following the UK debut of Nightmare in Silver a new and exclusive prequel for the next episode – the series final, The Name of the Doctor – will be released.
The site says that the minisode, written by Steven Moffat is:
One of the show’s longest prequels and explores how little Clara knows about the Doctor… and vice versa.
The video will be posted here directly after it’s release, although be warned for those outside the UK, it may feature spoilers pertaining to Nightmare in Silver.
UPDATE: Watch it now.