Tooth chews or grinds primarily (3)
I believe the answer is:
cog
'tooth' is the definition.
(cog is a kind of tooth)
'chews or grinds primarily' is the wordplay.
'primarily' suggests taking the first letters.
The initial letters of 'chews or grinds' is 'COG'.
(Other definitions for cog that I've seen before include "Small gear-wheel" , "Minor person in organisation" , "Each of series of teeth on wheel" , "Tooth on the rim of gear wheel" , "minor employee" .)