I'd almost go along with War Pigs suggestion Mr Kurtz but tell us whereabouts down the butt the bloody crack is! If it's outside of the butt plate screws, you could glue it. If it's inboard of the screws then best to cut-out and patch as has been done by a million Armourers since time immemorial. If it's in the centre of thge butt, along the line of the stock bolt recess, don't bother. It's beyond help