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.
This could also refer to an Austin Powers reference. In Austin Powers, his mojo is the source of his sex drive. It's possible that this is what the Doctor thought she meant, which would explain his response.