It's a children's book. In the underground world, everyone's names start with the same letter and there is a king. I remember they were not nice people. She goes above ground and meets a boy, and they have to save the world by stopping and starting time. At the end she gets a new name, and it's "Mophy," or "Sophy," or something like that.
I read this in Australia as a child (around 2008). I remember the cover had a large clock/timepiece, and there were a lot of colours, including gold.