A moving part, albeit mechanical (3)
I believe the answer is:
cam
'a moving part' is the definition.
'cam' can be an answer for 'part' (I have seen 'Part of engine' mean 'cam' so perhaps 'part' could also mean 'cam'). I'm not sure about the 'a moving' bit.
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for cam that I've seen before include "Projection on a rotating shaft in contact with another part" , "Rotating projection; river" , "Reciprocal-motion part" , "Part of engine" , "Rotating mechanism" .)