It being related to music is the only correct part of the last definition. A level reached by few, overground is when an underground artist becomes so popular that they can no longer be called underground. I would say that any underground artist who sells 1 mill+ copies of any one of their albums can be considered overground.
Tech N9ne, Tecca Nina, and Aaron Yates are the only overground artists that come to mind. . . they are all the same person if you didn't catch that.