I would love to know the same thing. It's amazing to me that Microsoft wouldn't include such a basic feature in ON. The Screen Clipping feature is fine if you only want to copy what's on your display, but it's useless if you have a larger section to deal with. I heavily rely on the "print to ON" function to import pages from online textbooks. But I don't always need everything on certain pages. It'd be really nice to be able to "crop" out what I don't need instead of drawing a big "X" over it all with my ON pen!
Let me know if you hear of any good workarounds.