heir apparent 
noun (plural heirs apparent)
1An heir whose claim cannot be set aside by the birth of another heir. Compare with heir presumptive.The Emperor is on his throne and the heir apparent, the Prince, is one step below....
- Prince Charles, now the Prince of Wales, heir apparent to the throne, was born in 1948, and his sister, Princess Anne, now the Princess Royal, in 1950.
- Why should the heir apparent be guaranteed the succession?
1.1A person who is most likely to succeed to the place of another: he was once considered heir apparent to the chairman...
- Mr Portillo was once considered the heir apparent to Mrs Thatcher.
- He was once considered the heir apparent to the throne but now he is a fighter searching for a career.
- Now, Tiger is once again the heir apparent and columnists across the globe are making the kinds of U-turns only journalists can get away with.