US soldier in car or rising aircraft (8)
I believe the answer is:
autogiro
'aircraft' is the definition.
(autogiro is a kind of aircraft)
'us soldier in car or rising' is the wordplay.
'us soldier' becomes 'GI' (GI can mean an American soldier).
'in' indicates putting letters inside.
'car' becomes 'auto' (automobile).
'rising' says the letters should be written backwards (in a down clue, letters go up).
'or' backwards is 'ro'.
'auto'+'ro'='autoro'
'gi' placed into 'autoro' is 'AUTOGIRO'.
(Other definitions for autogiro that I've seen before include "Self-propelled aircraft" , "that's whizzing up" , "Aircraft with free rotor" , "Aircraft supported by freely-revolving rotors" .)