Mojo

Look it up at Wiktionary.

A mojo was among the things that Clara Oswald lost as a child. She eventually got it back. Exactly what a mojo was was not specified. (WC: The Bells of Saint John: A Prequel)

Behind the scenes

This could refer to the magazine MOJO, a music magazine that has been published in the UK since 1993.

Clara could also be using mojo as slang. To have "lost your mojo" refers to a loss of inspiration or creative genius, the loss of that special spark. Mojo can be also be a reference to charm or sex drive, a meaning heavily popularised by the Austin Powers series. Since the Austin Powers films were released during Clara's childhood, she may very well have been aware of them when she made this reference.