ooooh….that’s tough. Chiffon is a tough fabric to work with when it comes to repairs because it just frays so much. If it was me, I would apply fraycheck to affected area, just to contain the damage and strengthen it, maybe do it twice (couldn’t hurt). After it was dry, cut off the random fibers to get it as neat as possible. If you can, undo the seam and try and resew it. Or you can try to hand sew it to the current seam, make your stitches small to make it less noticeable. Let me know how it turns out for you.