Two choices. Fill it with epoxy or CA glue or fill it with wood. If you choose to fill it, probably the easiest is the dark brown CA glue. Just put on spray with accelerator and sand off smooth. This is best for small defects. Yours looks to be an okay size to fill. Usually this looks very good when finished.
Option two is to fill with wood. This is a little more tricky but is a really nice option. The key is to get really nice tight joints along the patch. Regular wood glue will blend in the best.
Option two is to fill with wood. This is a little more tricky but is a really nice option. The key is to get really nice tight joints along the patch. Regular wood glue will blend in the best.