Briefly anointed barbarous Boris Johnson? (7)
I believe the answer is:
etonian
'boris johnson?' is the definition.
'briefly anointed barbarous' is the wordplay.
'briefly' means to remove the last letter.
'barbarous' indicates anagramming the letters.
'anointed' with its final letter taken away is 'anointe'.
'anointe' with letters rearranged gives 'ETONIAN'.
(Other definitions for etonian that I've seen before include "Public schoolboy" , "Public school boy" , "student in establishment?" , "Pupil at posh English school" , "Public school pupil" .)