RE: I dont think it is the gear mesh.
The worst that could happen if you leave it alone is a stripped spur. I would recommend replacing it sooner than later, just so you don't screw anything else up. As for a pebble or something getting in there, its possible. I think however that there was an issue with your clutch bell coming loose or something to that effect. That is how I chewed up my spur a few weeks ago.