Be about to lead one sultanate (6)
I believe the answer is:
brunei
'sultanate' is the definition.
(Brunei is an example)
'be about to lead one' is the wordplay.
'about' indicates putting letters inside.
'to lead' becomes 'run' (eg to run a company is to lead it).
'one' becomes 'i' (Roman numeral).
'be' placed around 'run' is 'brune'.
'brune'+'i'='BRUNEI'
(Other definitions for brunei that I've seen before include "Its sultan is famously very wealthy" , "Sultanate of southeast Asia" , "Muslim state" , "major source of oil" , "Far Eastern sultanate" .)