In advance of offer, one claims throne (9)
I believe the answer is:
pretender
'one claims throne' is the definition.
(someone who claims a right to the throne)
'in advance of offer' is the wordplay.
'in advance of' becomes 'pre' ('pre' means before).
'offer' becomes 'tender' (tender can mean to offer something).
'pre'+'tender'='PRETENDER'
(Other definitions for pretender that I've seen before include "Claimant to a throne, usually without just title" , "One acting" , "One claiming a title" , "Claimant, like Bonnie Prince Charlie" , "One who claims a title or position" .)