Not crop, but you could animate a garbage matte.
If you mean like making a hole: use a reversed track matte (shapes can be made in the titler) or the circle effect.
What is it you want to achive exactly?
What do you wish to see "through" that "hole?"
If Video, or a Still Image, would a PiP (Picture in Picture) not accomplish that?
Track Mattes are good for shapes and gradients. Just use a Photoshop .psd with transparency and the track matte can apply that to a video source.
You can also use your title maker to create a matte image for a track matte.
Can you be more specific about waht you are trying to do?
I've been using the reverse track matte method and it has worked without issue. One of the systems we have is a bit under powered and I was trying to cut down on its need to render. I wanted to be sure I wasn't missing some obvious toggle that would accomplish the same thing.
Thank you all for your quick responses.