If it's not deep, just a hairline at the surface - smooth on some epoxy, just basically to seal it. The 5 min stuff you can get at the local hardware store will do the trick, but it will dry brown'ish. You can get RR epoxy that will dry clear, but it's expensive and you'll end up with a ton left over. The only other option is to get some dye to change the color of the epoxy.
If it's deep, you'll want to fill it in with a mix of epoxy and caobosil or microballoons. Then a small patch of fiberglass over the top to keep it from splitting further.
TAP Plastics carries most everything you need and a search online for 'fiberglass' should turn up online sources for you, along with instructions for use of the product you choose.