Sultanate supporting the old guard (6)
I believe the answer is:
yeoman
'guard' is the definition.
(I know that yeoman is a type of bodyguard)
'sultanate supporting the old' is the wordplay.
'sultanate' becomes 'oman' (Sultanate of Oman, Middle Eastern country).
'supporting' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'the old' becomes 'ye' (resembles a historical spelling of 'the').
'oman' put after 'ye' is 'YEOMAN'.
(Other definitions for yeoman that I've seen before include "Land-owning farmer - beefeater" , "Former royal servant" , "Warder of the Tower of London" , "One may (anag.)" , "Historically, English freeholders and footsoldiers" .)