Part of engine unaffected by reversal (5)
I believe the answer is:
rotor
I believe this clue is a double definition.
'part of engine' is the first definition.
'unaffected by reversal' is the second definition.
(Other definitions for rotor that I've seen before include "Revolving component" , "Turning component" , "Copter blade assembly" , "Revolving arm in machine" , "Helicopter propeller" .)