Flight verges on the swiftest aviation route (8)
I believe the answer is:
stairway
'flight' is the definition.
(flights of stairs)
'verges on the swiftest aviation route' is the wordplay.
'verges on the swiftest aviation' becomes 'stair' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'route' becomes 'way' (way is a kind of route).
'stair'+'way'='STAIRWAY'
Can you help me to learn more?
(Other definitions for stairway that I've seen before include "A flight of fixed indoor steps" , "- to Heaven, Led Zeppelin song" .)