You just want to cut off part of a layer - not crop your whole document?
Hit C for the crop tool - that will crop the WHOLE document.
If you just want to crop parts of a layer down - you can draw a rectangle selection (M) around what you DON'T want and hit delete - or - a better way - draw a rectangle selection around what you DO want and hit the 'Add Layer Mask' button at the bottom of your layers pallette.
This doesn't erase anything - just hides it. It's a great technique.
Does that help?
// jayse
Ultimate After Effects Video Training CDs
"Essential AE Training"
by jayse!
