Firm gets rid of one exchanged for cash (4)
I believe the answer is:
sold
'exchanged for cash' is the definition.
(to sell is to exchange for cash)
'firm gets rid of one' is the wordplay.
'firm' becomes 'solid' (synonyms).
'gets rid of' suggests deleting specific letters.
'one' becomes 'I' (Roman numeral).
'solid' with 'i' taken away is 'SOLD'.
(Other definitions for sold that I've seen before include "Betrayed for money" , "Flogged" , "Handed over for money" , "Persuaded" , "Pushed" .)