I will see if I have a stock lying about with a crack in it to photograph what I am going to try to explain.
Remove the action from the stock, of course, so you can see what you are up against.
You may have to carefully clean the wood in there with a volatile solvent, like lacquer thinner, if there is oil in the wood, Do not get it on the finish or it will dissolve it.
Now, let's say the crack is on top, at the tang, but not all the way through. That probably indicates that you need to bed the recoil lug, too, because something is allowing the rear of the receiver to pound into the wood. The inletting may have been a bit too tight on the sides, and if there is a little taper, the tang will drive in and wedge the wood apart. So you may need some epoxy bedding there, too.
At any rate, to get some epoxy down in the crack, drill a tiny ( 1/64 or 1/32 at most ) hole into the crack, from where the steel is bedded. If the crack is long, you may need to drill a 1/64 hole in the grain line on top of the grip, where the end the crack is headed, to interrupt the crack and relieve stress. You can mask off the wood with Scotch tape and pack some epoxy in there with a pin.
Mask off all the wood with Scotch painter's tape, and press it down well. Trim it with a razor knife.
Pack epoxy in the other hole you drilled. If the inletting is 1/2 inch deep or more, drill two holes, one at the bottom and one halfway up. Push the epoxy in there with your finger. A hypodermic needle would be great, if you can get one. Then clamp it. A really small bar clamp with a trigger and rubber pads on the jaws, that swivel, is what you need, so you don't dent the wood. Be gentle. Let it sit 24 hours.