Successor clearly seen (4,8)
I believe the answer is:
heir apparent
'successor' is the definition.
(thesaurus)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for heir apparent that I've seen before include "One expected to succeed" , "Acknowledged beneficiary" , "sure of inheritance" , "Next in line to throne" , "successor eventually" .)