Fish dish ultimately going wrong (7)
I believe the answer is:
herring
'fish' is the definition.
(I know that herring is a type of fish)
'dish ultimately going wrong' is the wordplay.
'ultimately' suggests the final letters.
'going wrong' becomes 'erring' (I've seen this in another clue).
The last letter of 'dish' is 'h'.
'h'+'erring'='HERRING'
(Other definitions for herring that I've seen before include "It's certainly fishy!" , "Common food fish" , "Saltwater fish" , "Important food fish" , "Fish that's kippered" .)