Part of engine restricted by zero torque (5)
I believe the answer is:
rotor
'part of engine' is the definition.
'restricted by zero torque' is the wordplay.
'restricted by' indicates a hidden word.
'ROTOR' is hidden within 'zero torque'.
(Other definitions for rotor that I've seen before include "Revolving component" , "Palindromic revolving part of machine" , "part of chopper" , "Helicopter lifter" , "Palindromic moving part" .)