Copter part (5)
I believe the answer is:
rotor
'copter part' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for rotor that I've seen before include "Turner" , "Copter blade assembly" , "Mechanical turning part" , "Part of, eg, a motor" , "It goes round and round (and back and forth)" .)