The majesty of King George under a doctor (8)
I believe the answer is:
grandeur
'the majesty' is the definition.
(thesaurus)
'king george under a doctor' is the wordplay.
'king george' becomes 'gr' (Georgius Rex).
'doctor' indicates an anagram.
'under'+'a'='undera'
'undera' is an anagram of 'andeur'.
'gr'+'andeur'='GRANDEUR'
'of' is the link.
(Other definitions for grandeur that I've seen before include "The splendour of France" , "Magnificence - rare dung (anag)" , "Nobility, dignity" , "Majesty (God's, Hopkins)" , "Impressive splendour" .)