Clear Crop will remove the crop so that you get the biggest possible image.
Set to Original Crop will reset the crop to the default (or "original") crop. For some cameras, these two are actually the same.
For other cameras, however, there are in-camera aspect ratio settings (3:2, 4:3, 1:1, 16:9, etc.) which can be used to set the default crop. This will be easier to explain with an example. Let's say you're shooting with a Canon 6D and set the in-camera aspect ratio to 1:1. When you bring the raw file into ACR/Lr the default crop will be a 1:1, which is a square crop from the full 3:2 image. If you choose "Clear Crop" you will get the full uncropped 3:2 image. If you choose "Set to Original Crop" you will return to the 1:1 crop. And of course, you can change the crop to whatever you like.
One more note: some camera models (e.g., Panasonic and Nikon models) offer in-camera aspect ratios but these settings actually crop the raw data itself. Choosing Clear Crop in these cases won't give you back the full 4:3 or 3:2 image.
Thank you ...
Then, I think that "original" option in LR means "clear crop" in ACR ... and "as shot" option in LR means "set to original crop" in ACR.
Why don't you use same terminology for LR and ACR?
Your point is well taken. Yes, we should have used the same terminology. (I tried ... !)