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 Mojo magazine", a music magazine that has been published in the UK since 1993.
Clara could also be using "mogo" 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. This meaning was popularised by the Austin Powers movies where his mojo is the source of his sex drive. The Austin Powers movies were released during Clara's childhood, so she may have been aware of them when she made this reference