Rotating part (3)
I believe the answer is:
cam
'rotating part' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for cam that I've seen before include "Rotating part in machine; river" , "Bump on a shaft for pushing another shaft" , "Projection on a rotating shaft in contact with another part" , "Projection on rotating cylinder" , "Rotating mechanism" .)