Behind the scenes
While the exact meaning of "mojo" was not specified in the episode, Clara was most likely using mojo as slang. To have "lost your mojo" refers to a loss of inspiration or creative genius, the loss of that special spark.
However, mojo could also refer to the magazine MOJO, a music magazine that has been published in the UK since 1993. 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, though likely with only a child's understanding of the term.