Firm German wheel (3)
I believe the answer is:
cog
'wheel' is the definition.
(wheel with teeth in a machine)
'firm german' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'german' becomes 'g' (abbreviation).
'co'+'g'='COG'
(Other definitions for cog that I've seen before include "minor employee" , "Machine component" , "Small gear-wheel" , "One has teeth" , "Each of series of teeth on wheel" .)