There are no direct covert actions that will let you kill your heir. However, do not lose hope, for there is still a chance at murder.
- There are a few random events that can either: a) let you choose an option that kills your heir or, b) simply kills your heir (the hunting accident event, for example)
A much more direct approach:
- You can assign your heir to become a general in the Military tab. Then, put him in command of a thousand brave troops and send him to war against an overwhelming enemy. Repeat until he is dead.