Is there an option to do that? Or simply to put the clip on your computer? I’d be surprised if there wasn’t an option for that.
Think.
What is a clip?
And what is a theater file?
They’re not the same thing.
No there isn’t, unless you capture it yourself.
Gotta render it first.
You need a capture card to do so.
you can to to http://halorenders.com/ and they will help you with the clip 
I’m pretty sure you would need a device to record them onto your computer first.
Here’s a pretty good place to start: http://jacobsmalls.hubpages.com/hub/Guides-to-Recording-Xbox-360-Gameplay-to-Desktop-or-Laptop
Thanks for those links everyone, especially gonna go to that Halo render site, see if I can get the clips through there.