Advantage of superior agency (5,4)
I believe the answer is:
upper hand
'advantage' is the definition.
(upper hand is a kind of advantage)
'superior agency' is the wordplay.
'superior' becomes 'upper' (eg an upper house in a parliament).
'agency' becomes 'hand' (I've seen this in other clues).
'upper'+'hand'='UPPER HAND'
'of' acts as a link.
(Other definitions for upper hand that I've seen before include "Superior position" , "superiority" , "Position of advantage and control" , "not usually right for driver" , "Mastery, advantage" .)