You just need to get some of the thin (thinnest-thinner than water) superglue and get it in there. Nothing from the grocery store will do, but hobby shops have it. They also have "accelerator" which speeds the drying time and increases the bond strength. You don't need the sped up dry time, but you do want the extra strength. So first, clean the crack with peroxide or alcohol, and let dry completely. Then spray the area with the accellerator, then wipe it off. Wait about 30 seconds, and then put the glue in there. Watch around the other side that it doesn't come flooding out the truss rod nut area. You don't want glue in there. It would be nice to see a little moisture come through so you could tell it went all the way through. If you can see the crack from inside the truss rod adjustment area, then seal that part off first, or better yet, fill entirely from that side, so the glue runs out the back of the neck. But I repeat, you don't want to glue the truss rod nut or the rod itself in place. Then you can scrape the extra glue away with razors and finish off with flat files and sandpaper. It will be better than new, since the superglue will have soaked the area and hardened in there.