Flying vehicle on flight, finally, for Mars? (6)
I believe the answer is:
planet
'mars?' is the definition.
(mars is a kind of planet)
'flying vehicle on flight finally' is the wordplay.
'flying vehicle' becomes 'plane' (aircraft).
'on' says to put letters next to each other.
'finally' suggests the final letters.
The final letter of 'flight' is 't'.
'plane'+'t'='PLANET'
'for' acts as a link.
(Other definitions for planet that I've seen before include "Orbiting body in the heavens" , "Jupiter is one" , "Earth, for instance" , "Celestial body orbiting around a star" , "Celestial body orbiting round a star" .)