Hair line cracks are tough to fix. Does the crack spread apart any or is it just a visable hairline?
If its just a visible hair line, I'd try super glue, find a hobby shop, what you want is Hot Stuff Cyanoacrylate, the glue has the conistancy of water, its very thin and has a strong bond. Be warned this is not the super glue you buy in the supermarket!
I'd remove the finish along the crack line. Apply the glue carefully along the crack, capilary action will pull the glue it into the crack and the surrounding wood. Clamp if possible and let sit for twenty four hours +. Then apply some finish over the crack line.
If the crack spreads open then I'd use wood glue. with the crack spread apart work glue into the crack with a tooth pick, a thin piece of wire, xacto knife blade, etc. If you have a air compressor (or get a can of compressed air at radio shack) use that to blow the glue into the crack. Then clamp and remove the excess glue with a damp cloth. Wait 24+ and then apply finish over the crack.
