Agree to be heir to? (6)

I believe the answer is:

accede

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'agree to' is the definition.
(thesaurus)

I don't understand how the remainder of the clue works.

(Other definitions for accede that I've seen before include "Agree to what is proposed" , "'Take office, as king say (6)'" , "Yield" , "Grant, agree to proposal" , "Agree (to) - come into office" .)

I've seen this clue in the Evening Standard.

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