A diner's ordered fish (7)
I believe the answer is:
sardine
'fish' is the definition.
(sardine is a kind of fish)
'a diner's ordered' is the wordplay.
'ordered' is an anagram indicator.
'a'+'diners'='adiners'
'adiners' is an anagram of 'SARDINE'.
(Other definitions for sardine that I've seen before include "A diner's small fish" , "(Cramped?) fish" , "Fish in tin?" , "Fish (often tinned)" , "Small fish, usually canned" .)