Serving up the same cold fish (3)
I believe the answer is:
cod
'fish' is the definition.
(cod is a kind of fish)
'serving up the same cold' is the wordplay.
'serving up' shows that the letters should be reversed in order.
'the same' becomes 'do' (abbreviation for ditto).
'cold' becomes 'c' (eg on taps).
'do'+'c'='doc'
'doc' backwards is 'COD'.
(Other definitions for cod that I've seen before include "mock" , "Initially, pay when you receive the goods" , "Hoax (sl.); fish" , "Cash on delivery (abbr.)" , "A fish; a hoax, parody" .)