Aircraft without an engine (6)
I believe the answer is:
glider
'aircraft without an engine' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for glider that I've seen before include "Plane without an engine" , "Powerless aircraft" , "Aircraft designed to fly without an engine" , "Kind of aircraft without an engine" , "Flying machine without engine" .)