For simple clipping, you can put your element inside of a group, change the group’s Content Overflow settings in the Metrics inspector to hidden, and then adjust the bounding box of the group to mask off part of the contained element.
See this post:
There’s a few other interesting methods here:
More complicated clipping paths can be done manually with SVG, but aren’t built into Hype. Here’s a post that has some of that info: