The other day I replaced the bell tenon on a clarinet. First I used the lathe to cut off the old tenon so it was flush with the clarinet body. Then I used a cutter to bore the right size and depth hole into the body to fit the replacement tenon. Finally, I glued the new tenon in with epoxy. This was a replacement tenon sold by Votaw Tool. You could manufacture a tenon yourself from plastic stock but its much easier to just buy one if you can.
This is a plastic clarinet, on a wood clarinet it would be more difficult to repair a broken tenon. Not impossible, but would be much more time consuming.