I'm assuming the crack is vertical, starting at the tang, and extending toward the butt. Remove the stock. Working from the inletting, drill in and open out across the crack with a Dremel and a small burr. Don't go near the surface. Pack this cavity with epoxy or bedding compound. Sometimes a little roughened steel pin can be pushed into the cavity to bridge the crack mechanically. If the crack can be exposed with a bit of side pressure, it might be possible to get some epoxy down into the crack. Or, with the epoxy in the cavity, by springing the crack open and closed, the epoxy may be drawn into the crack. Don't use enough force to make the crack larger, just enough to draw the epoxy into it. Make sure none is left on the surface. Wrap the stock with elastic while the repair cures. If oil has got down into the crack, the epoxy may not adhere well to the wood. This is why working inside, dentist style, can stabilize the crack. Use a slow cure quality epoxy, not 5 minute type. Its stronger. Bedding compound, like Acraglas, Acraglas Gel, or Microbed can also be used.