Firm with five hundred fish (3)
I believe the answer is:
cod
'fish' is the definition.
(type of fish)
'firm with five hundred' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'with' says to put letters next to each other.
'five hundred' becomes 'D' (Roman numeral).
'co'+'d'='COD'
(Other definitions for cod that I've seen before include "Hoax (sl.); fish" , "Cash on delivery, in short, for this fish" , "Kid" , "swimmer" , "Pull the leg of - a fish?" .)