Ate cod up, then king fish (7)
I believe the answer is:
haddock
'fish' is the definition.
(haddock is a kind of fish)
'ate cod up then king' is the wordplay.
'ate' becomes 'had' (e.g. I had an ice cream).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'then' says to put letters next to each other.
'king' becomes 'K' (abbreviation for king in chess).
'cod' backwards is 'doc'.
'had'+'doc'+'k'='HADDOCK'
(Other definitions for haddock that I've seen before include "Cod-like fish" , "N. Atlantic food fish" , "Popular food fish" , "Fish like cod but smaller" , "White fish" .)