Liver oil source (3)
I believe the answer is:
cod
'liver oil source' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for cod that I've seen before include "Initially, pay when you get the goods" , "Fish; fake (colloq.)" , "Tasty fish or slangy nonsense" , "Common food fish" , "Fish noted for liver oil" .)