Tooth on a gearwheel (3)
I believe the answer is:
cog
'tooth on a gearwheel' 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 "Junior official" , "that engages with others in revolution" , "Each of series of teeth on wheel" , "One has teeth" , "Minor person in organisation" .)