A reciprocating mechanical part (5)

I believe the answer is:

rotor

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'part' is the definition.
(I have seen 'part of chopper' mean 'rotor' so perhaps 'part' could also mean 'rotor')

I don't understand the rest of the clue.

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(Other definitions for rotor that I've seen before include "Device turning" , "Turning component" , "Turning part of an engine" , "Revolving part of machinery" , "one lifting chopper?" .)

I've seen this clue in the Evening Standard.

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