Diners upset about a fish (7)
I believe the answer is:
sardine
'fish' is the definition.
(sardine is a kind of fish)
'diners upset about a' is the wordplay.
'upset' indicates an anagram.
'about' means one lot of letters goes inside another.
'diners' is an anagram of 'srdine'.
'srdine' enclosing 'a' is 'SARDINE'.
(Other definitions for sardine that I've seen before include "Swimmer" , "A diner's small fish" , "Fish (often tinned)" , "(Cramped?) fish" , "Small pilchard" .)