Part of a gear (3)
I believe the answer is:
cog
'part of a gear' is the definition.
('cog' is part of 'gear')
This is all the clue.
(Other definitions for cog that I've seen before include "Each of series of teeth on wheel" , "Tooth on gearwheel" , "Junior official" , "Wheel with projections" , "Small gear-wheel" .)