Flight route on a flight route (8)
I believe the answer is:
stairway
'flight' is the definition.
(flights of stairs)
'route on a flight route' is the wordplay.
'route' becomes 'st' (abbreviation for street).
'on' means one lot of letters go next to another.
'a flight route' becomes 'airway'.
'st'+'airway'='STAIRWAY'
(Other definitions for stairway that I've seen before include "A flight of fixed indoor steps" , "- to Heaven, Led Zeppelin song" .)