var canvas = new fabric.Canvas('canvas')

var rect = new fabric.Rect({
  width: 100,
  height:100,
  fill: 'red',
  left: 100,
  top: 100
});

canvas.add(rect)

var clipGroup = new fabric.Group([
 new fabric.Circle({
    radius: 50,
    left: -50,
    top: -50
  }),
  new fabric.Rect({
    width: 25,
    height: 25,
    left: 0,
    top: 0,
    globalCompositeOperation: 'destination-out'
  })
]);

rect.clipPath = clipGroup;