Small firms take in cash (5)
I believe the answer is:
coins
'cash' is the definition.
(coins are cash)
'small firms take in' is the wordplay.
'small firms' becomes 'cos' (co is short for company - a business or firm; 'small' asks for a short form).
'take' means one lot of letters goes inside another.
'cos' going around 'in' is 'COINS'.
(Other definitions for coins that I've seen before include "change" , "Metal money" .)