That's nasty and not an easy fix if you want it to look nice. It looks to me like the hole for the cleaning kit, which I assume is what is inside the opening, was drilled off center and the wood in this area is VERY thin. Because of that you might need to consider making a plug of matching wood and closing the hole in the stock before you can graft in a slip of wood to fix the hole.
If you try to graft in a slip of wood as it is without filling the hole in the stock I'd be worried that you're just setting yourself up for a second break sooner instead of later.