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' indicates putting letters inside.
'to lead' becomes 'run' ('run' can be a synonym of 'lead'**).
'be'+'i'='bei'
'bei' enclosing '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" .)