Chopper part (5)
I believe the answer is:
rotor
'chopper part' is the definition.
('rotor' is part of 'chopper')
This is all the clue.
(Other definitions for rotor that I've seen before include "device for lifting a chopper" , "Turning part of an engine" , "Helicopter propeller" , "Arm moving" , "Revolving part of machinery" .)