Firm in the money (4)
I believe the answer is:
coin
'the money' is the definition.
(I've seen this before)
'firm in' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'co'+'in'='COIN'
(Other definitions for coin that I've seen before include "Mint" , "little money" , "Invent a word or phrase - for money?" , "Create (phrase); metal token" , "Money; devise (a word)" .)