Mechanical part of a camera (3)
I believe the answer is:
'mechanical 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 'mechanical' bit.
I cannot 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 "It controls the motion of engine cylinder valves" , "Projection on rotating shaft imparting motion" , "Part of engine" , "River of eastern England" , "River; shaft projection" .)