1. A concise explanation of the problem you're experiencing.
The label is split with the background, and the billboard is not clamped to ground.
2. A minimal code example. If you've found a bug, this helps us reproduce and repair it.
3. Context. Why do you need to do this? We might know a better way to accomplish your goal.
I need clamp to ground to work so that I can clamp things to the ground.
4. The Cesium version you're using, your operating system and browser.
Cesium 1.38. OSX 10.12.6. Chrome 62.0.3202.38.
Tested on Safari 11.0 as well and it seems to work. May be an issue with Chrome on OS X?