The Great Clock was a massive structure of gears, chains and pendulums that kept time. It included the same technology as the house in Ely, feeding on the minds of prisoners who were enslaved and maintained the clock. Because of this, unprotected people in the Clock could be affected mentally. There were vast amounts of heat, making the Clock seem like a furnace.
Because it was so massive, the Great Clock warped the space-time around it and served as a counter-weight for the hyper lanes that were the key to the Earth Empire's power. This was the great secret passed down to each Guardian of the Solar System. When he became Guardian, Mavic Chen sought to free the prisoners and still maintain the Earth's power without the Great Clock, and spent half a century gathering minute traces of taranium in the hopes it could achieve this goal.
In 3999, a future Sara Kingdom, when her mind was attacked by the Clock, destroyed it, causing the structure to collapse and setting in motion Chen's alliance with the Daleks to ensure Earth's power would remain. (AUDIO: The Guardian of the Solar System)