In reply to idiotproof (Buxton MC):
> I was thinking sew it and then maybe a patch behind sealed with tent repair stuff?
Align the edges as best you can, then a zig-zag stitch along the three sides.
Whether it then needs tape reinforcing will depend on the nature of the fabric. If it's a bonded laminate, then the lamination and stitching is likely to be enough to keep all the threads together and stop it fraying. If it's a non-laminate fabric, then maybe tape it to prevent fraying.
If it's a tape seamed waterproof 'soft shell', then maybe consider taping to maintain the waterproofing, but that will really depend on where the tear is; in the body, you're not likely to need that much waterproofing, but shoulders will.
Here's my guide to using iron-on seam seal tape, but spinnaker tape might be all that's needed:
http://www.outdoorsmagic.com/hill-skills/how-to-repair-torn-waterproofs/650...