Crossword Genius

I did it at Oxford writing to heir apparent (9)

Ross

I believe the answer is:

successor

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'heir apparent' is the definition.
(thesaurus)

I cannot understand how the rest of the clue works.

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(Other definitions for successor that I've seen before include "New leader" , "One taking over from another" , "one will take your place" , "I follow" , "Heir" .)

I've seen this clue in the Irish Times and the Irish Times (Crosaire).
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