MacDraw Pro does not have the ability to Crop or Mask images, whether drawn in MacDraw Pro or imported from another application.
Workaround:
Try using white boxes to cover the portions of the graphic that you want to Crop. This is not a good method if you are trying to Crop an image for use in another program. Consider another program for editing images such as BrushStrokes from the Claris Clear Choice division of Claris.