You mean the moulded section does not bear against the fuse end, right? That is an issue with the fitting and you will need to sand it in place. Or else, thicken some epoxy with filler (microballoons, saw dust, even talcum powder) and then apply. You will need a 50% or more mixture of filler to prevent from running. If you can reach inside, tape the inside of the crack with some masking tape and then fill. You can remove the mess with acetone dabbed cloth. Remember to not apply the acetone on the decals, only on the monokote

Few tips to reinforcing this airplane in my earlier posts, have a look

Hope this helps

Ameyam