JMOR is correct, and the proper repair is to replace parts. That said, you *may* be able to tighten it up by getting an appropriate sized washer to make up for the wear. The washer in question will need to have a big enough ID to slip over the shaft and press on the hub.
If you try the washer trick and it appears to work, don't just tighten it down once and move on with life. Drive it around and tighten it again, and keep doing that multiple times until the nut stays tight. If the nut stops moving and stays tight, you should be good to go.