Definitions of heir presumptive
someone who seems likely to take over a job or a position when someone else leaves that job or position
Lachlan - until now generally seen as the heir presumptive - appears to have ruled himself out of the race.
someone who will, or who seems likely to, inherit property or a title if a person dies at this moment, but whose inheritance may be altered in the future
Princess Elizabeth was by then heir presumptive to the throne.