Rubbish old Republican is part of machine (5)
I believe the answer is:
rotor
'part of machine' is the definition.
'rotor' can be an answer for 'part' (I have seen 'part of chopper' mean 'rotor' so perhaps 'part' could also mean 'rotor'). I'm unsure of the 'of machine' bit.
'rubbish old republican' is the wordplay.
'rubbish' becomes 'rot' (I've seen this before).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'republican' becomes 'r' (abbreviation).
'rot'+'o'+'r'='ROTOR'
'is' is the link.
Can you help me to learn more?
(Other definitions for rotor that I've seen before include "Part of engine that revolves" , "System of turning helicopter blades" , "Revolver" , "Helicopter lifter" , "Part of, eg, a motor" .)