Hi,
I used to build custom rods for myself and my friends. The best way to repair that damage is to first buy a cork ring that is used for building the rod handle. Carefully cut out the area that is damaged. Measure the height and width of the cork ring to fit in the cavity that was cut out. Make it fit tight and bigger than the handle so that you can sand it down to blend It into the original handle. I would use 5 minute epoxy ( be sure to mix it well) to glue it in place because when it’s dried, it’s almost transparent. Be sure to apply the epoxy to all the surfaces that needs to be glued together. It’s important to wipe away any excessive epoxy that Is squeezed out so that it does not show when it’s finished. I use painters tape to hold the added piece in place or you could just hold it down with you fingers until it dries. Let it dry completely before attempting to sand it down. For sanding, I would use a fine wood file to shape it and file it down to size, then finished with extra fine sandpaper. This should solve your problem and you will barely notice the repair. Good luck.