Be one about to lead nation (6)
I believe the answer is:
brunei
'nation' is the definition.
'be one about to lead' is the wordplay.
'one' becomes 'i' (Roman numeral).
'about' means one lot of letters goes inside another.
'to lead' becomes 'run' (eg to run a company is to lead it).
'be'+'i'='bei'
'bei' placed around 'run' is 'BRUNEI'.
(Other definitions for brunei that I've seen before include "Eastern land" , "It's sultan is immensely rich" , "Rue bin in the sultanate" , "Muslim state" , "State on Borneo" .)