Marine food fish (4)
'marine food fish' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for tuna that I've seen before include "swimmer" , "Skipjack --, blue-fin --" , "Popular food fish" , "Large salt water fish" , "Species of fish" .)
(Other definitions for cod that I've seen before include "One caught in the deep maybe" , "method of payment" , "Fish noted for liver oil" , "Initially, pay when you get the goods" , "A white fish" .)