Firm German seafood (3)
I believe the answer is:
cod
'seafood' is the definition.
(cod is a kind of seafood)
'firm german' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'german' becomes 'd' (abbreviation for Deutschland ie Germany).
'co'+'d'='COD'
(Other definitions for cod that I've seen before include "Payment arrangement" , "Common food fish" , "Fish noted for liver oil" , "mock" , "catch!" .)