Determine the severity of the crack in your clarinet. If it is large, is on one of the tenons or on the bell, the only practical thing to do is replace the part that is damaged. You can order parts from a local music store.
Fix a small hairline crack in a tenon or the sockets by tightening the metal bands around the clarinet to prevent the crack from spreading. These small cracks won't affect the sound and it is best to monitor them and fix them only when the cracks have become large and affect the playability of the instrument.
Purchase a tenon cap and place it over the tip of the tenon if you see any cracking in the tenons. This will prevent future splitting.