Tooth on a gear wheel (8)
(Other definitions for sprocket that I've seen before include "Toothed wheel driven by chain" , "Wheel projection engaging with hole in, eg, film" , "Cog wheel" , "Tooth for driving mechanism" , "Wheel with projecting teeth" .)
'tooth on a gear wheel' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for cog that I've seen before include "One has teeth" , "Machine component" , "Tooth on the rim of gear wheel" , "Machine wheel" , "One on a gearwheel" .)