Odd place, how'd it happen?
Well, take the stock off, clean out the crack good, work in some gorilla glue or other high strength adhesive, and figure out some way to clamp the crack together while it dries. The location of that crack makes it tough to clamp together, so just practice methods until you find one that works, then go for the glue. Don't overlook surgical tubing.
There are kits for stock repair that include a drill and screw to help hold it together, but that is such a hard spot to work with, I don't see how you'd do it.
or get ahold of Umarex or Walther and see what a new stock would cost, you never know,,,,,
thats is my thinking on the subject, by cracky
good luck and let us know how it turns out.