Aviator at last seen in barrel roll (4)
I believe the answer is:
turn
'roll' is the definition.
(turn can mean to roll over)
'aviator at last seen in barrel' is the wordplay.
'at last' indicates one should take the final letters.
'seen in' indicates putting letters inside.
'barrel' becomes 'tun' (tun is a kind of barrel).
The final letter of 'aviator' is 'r'.
'r' inserted inside 'tun' is 'TURN'.
(Other definitions for turn that I've seen before include "Revolution by runt" , "Sour" , "Rotate - go" , "Transform - revolution" , "revolutionary act" .)