How to cut off these clouds on my camera?

A simple fix would be to change the viewport rect on your camera component. But I'm not sure if that's the solution you're looking for

Going off of what Simon said, you could also use multiple cameras. Have one camera render the sky/groud, and have the other camera solely dedicated to rendering the clouds by placing them in a different layer.

If you do this method, you will have to change the viewport rect of the second camera or else you will have the same problem.

Interesting.. although the black plane is an easier answer, that seems very interesting. I'll try to use that sometime.