I believe something is in development to make this easier (e.g. this thread and this thread and others), but in the meantime:
You can make the canvas transparent. When nothing is selected, you can change the canvas colour in the Page properties on the right, and make the colour transparent (shows as grey checkerboard).
If you clip to your freehand shape and then save as PNG with a transparent canvas, you end up with a PNG with transparency outside the freehand shape area.
Images are always rectangular, with or without transparent areas. (This is true in any software, although internal objects might show as non-rectangular during the working process.)