Take office or agree (6)
I believe the answer is:
accede
I believe this clue is a double definition.
'take office' is the first definition.
(thesaurus)
'agree' is the second definition.
(acceding is a kind of agreeing)
(Other definitions for accede that I've seen before include "'Acquiesce, defer (6)'" , "Agree - take the throne" , "Agree (to request)" , "bow" , "Grant, agree to proposal" .)