Plane control surface (7)
I believe the answer is:
aileron
'plane control surface' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for aileron that I've seen before include "Trailing-edge flap" , "Movable wing surface" , "Movable part of an aircraft's wing" , "Hinged flap on aircraft wing" , "Wing control flap" .)