Small part of a machine (3)
I believe the answer is:
cog
'small part of a machine' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for cog that I've seen before include "one engaged in revolutionary activity" , "Each of series of teeth on wheel" , "bar" , "One on a gearwheel" , "Machine wheel" .)