Gear is firm – grand! (3)
I believe the answer is:
cog
'gear' is the definition.
(thesaurus)
'firm grand' is the wordplay.
'firm' becomes 'co' (abbreviation for company).
'grand' becomes 'g' (G is short for grand as in a thousand).
'co'+'g'='COG'
'is' acts as a link.
(Other definitions for cog that I've seen before include "One has teeth" , "Tooth on the rim of gear wheel" , "Minor player" , "Little gearwheel" , "Tooth on the edge of wheel" .)