Revolving machine part (5)
I believe the answer is:
'revolving machine 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 "Revolving part of machinery" , "Revolving mechanism" , "Reversible piece of moving machinery?" , "Rotating part of machine" , "Revolving blade (as on helicopter)" .)