One guarding the old sultanate (6)
I believe the answer is:
yeoman
'one guarding' is the definition.
(I've seen this before)
'the old sultanate' is the wordplay.
'the old' becomes 'ye' (resembles a historical spelling of 'the').
'sultanate' becomes 'oman' (Sultanate of Oman, Middle Eastern country).
'ye'+'oman'='YEOMAN'
(Other definitions for yeoman that I've seen before include "(Of old) a free person cultivating his own land" , "One may (anag.)" , "Royal attendant" , "Former royal servant" , "Farming freeholder of old" .)