Clom was one of twenty seven planets stolen from space and time into the Medusa Cascade by the Daleks. It could not be found since the time inside the Cascade was off by a single second. The Twenty seven planets were to be used to create a reality bomb, which would destroy every universe and every life form, except those who were in proximity of the bomb, making the Daleks the only living beings in all of existence.
This plan was thwarted by The Tenth Doctor, Donna Noble, and the Clone Doctor, and Clom was returned to its rightful place in the galaxy.
Abzorbaloff: No! I'm not the swine; I spit on them! I was born on their twin planet.
The Doctor: Really? What's the twin planet of Raxacoricofallapatorius?
The Doctor: Clom?
The Doctor: "Clom's gone? Who Would want Clom?!"