Only the cod I ordered in it was second-rate (8)
I believe the answer is:
mediocre
'second-rate' is the definition.
(I know that second-rate can be written as mediocre)
'only the cod i ordered in it' is the wordplay.
'only' becomes 'mere' (I've seen this in another clue).
'ordered' indicates anagramming the letters.
'in it' means one lot of letters goes inside another.
'cod'+'i'='codi'
'codi' anagrammed gives 'dioc'.
'mere' going around 'dioc' is 'MEDIOCRE'.
'was' is the link.
(Other definitions for mediocre that I've seen before include "Come ride (anag.)" , "Of middling quality" , "Pedestrian, second-rate" , "Run-of-the-mill" , "Rather inferior" .)