Gear tooth (3)
I believe the answer is:
cog
'gear tooth' 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 "Tooth on the rim of gear wheel" , "Each of series of teeth on wheel" , "Little gearwheel" , "bar" , "One on a gearwheel" .)