It is a conventional prop but is being used as a pusher so the rotation is reversed.
Do up the first nut as tight as you would for conventional rotation, then with two spanners tighten the outer nut, holding the inner one still. Now flown a great many times with no problem.
Like Quoreng said, double nuts work. If it’s one of those prop nuts with the aerodynamic tip then just regular tightening should suffice. Just make sure that it’s tight, but not hulk tight because the shaft can shear off and you have to replace collet, don’t ask me how I know. In general though, I wouldn’t worry, especially since the legacy has landing gear. I’ve only had props loosen on reverse direction belly landers, but it can’t hurt to be safe.
CW threads are just not necessary in a plane. In a quad, the motor is starting and stopping many times a second to maintain stability. It's the starting and stopping that loosen the prop nut. Airplane motors don't change speeds nearly as much as a quad.
Just tighten the prop adapter firmly with a pliers and you will be fine.