Crack is most likely due to corrosion of the barrel nut swelling and then cracking it. Well, maybe with the force of tightening acting in that direction, which is not helping. First thing is to knock the nut out to relieve that constant pressure.
Then as an idea which may or may not work....
Open up crack and allow epoxy to seep in, then let it close again. Wipe of excess. That has glued it up and putting in a new nut (after it has dried) may be enough. But to be safe I would then grind out along the crack say 3mm wide and 1mm deep and fill with fibres (glass, carbon whatever) and epoxy, then sand back. That has then increased the width of the surfaces bonded together.
Other way. Whack out nut. Fill hole with wax Grind / cut out a vee (from top of pic - where the screw comes down into fin) Fill that big vee with fibres and a good epoxy like JB Weld Redrill hole from top Remove wax and insert new barrel nut
Update. This is how I did the repair. After cutting in a "V" and gluing, I dremelled in a 1mm x 5mm groove across the crack and filled it with epoxy and glass strands:
Sanded, another coat of resin, and sanded again - stronger than new I think.
