Firm good for machine gear (3)
I believe the answer is:
cog
'machine gear' is the definition.
(I've seen this in another clue)
'firm good' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'good' becomes 'g' (abbreviation).
'co'+'g'='COG'
'for' acts as a link.
(Other definitions for cog that I've seen before include "Machine wheel" , "Minor player" , "Little gearwheel" , "Junior official" , "Tooth on the rim of gear wheel" .)