Reversible machine part? (3)
I believe the answer is:
cam
'machine part?' is the definition.
(I've seen this in another clue)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for cam that I've seen before include "Runner" , "It controls the motion of engine cylinder valves" , "Projection on rotating shaft imparting motion" , "Shaft on a rotating part in machinery" , "University city's river" .)