Asian sultanate, British-administered, that is to the west (6)
I believe the answer is:
brunei
'asian sultanate' is the definition.
(I've seen this in another clue)
'british-administered that is to the west' is the wordplay.
'british' becomes 'br' (abbreviation for British).
'administered' becomes 'un' (I can't explain this - if you can you should believe this answer much more).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'to the west' says the letters should be written in reverse (letters written right to left or 'east to west').
'ie' written backwards gives 'ei'.
'br'+'un'+'ei'='BRUNEI'
Can you help me to learn more?
(Other definitions for brunei that I've seen before include "Oil-rich sultanate of Borneo" , "Oil-rich country of Southeast Asia" , "Southeast Asian sultanate" , "Commonwealth member" , "Rich sultanate in the South China Sea" .)