Mechanical part of a bridge? (3)
I believe the answer is:
cam
'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 can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for cam that I've seen before include "River; shaft projection" , "Cambridge river" , "East Anglian river" , "River also called Granta" , "Rotating mechanism" .)