Fish lovers do upset me, ultimately (5,4)
I believe the answer is:
dover sole
'fish' is the definition.
(type of fish)
'lovers do upset me ultimately' is the wordplay.
'upset' is an anagram indicator.
'ultimately' indicates one should take the final letters.
The final letter of 'me' is 'e'.
'lovers'+'do'='loversdo'
'loversdo' anagrammed gives 'doversol'.
'doversol'+'e'='DOVER SOLE'
(Other definitions for dover sole that I've seen before include "Kentish flat fish" , "Catch" .)