A victory -- one swallowed fly (6)
I believe the answer is:
aviate
'fly' is the definition.
('aviate' can be a synonym of 'fly')
'a victory one swallowed' is the wordplay.
'victory' becomes 'v'.
'one' becomes 'i' (Roman numeral).
'swallowed' becomes 'ate'.
'a'+'v'+'i'+'ate'='AVIATE'
(Other definitions for aviate that I've seen before include "Pilot a plane" , "Handle jumbo, maybe" , "Fly" , "Ride the Zephyr, say" .)