One patronising royal house perhaps as poor — agree to differ (9)
I believe the answer is:
operagoer
'one patronising royal house perhaps' is the definition.
(an operagoer might patronise the Royal Opera House)
'poor agree to differ' is the wordplay.
'to differ' indicates anagramming the letters.
'poor'+'agree'='pooragree'
'pooragree' is an anagram of 'OPERAGOER'.
'as' acts as a link.
(Other definitions for operagoer that I've seen before include "One attending, eg, Covent Garden" , "attender of musical performances" , "One attending music theatre" , "One enjoying Carmen, say" .)