A diner's chewed fish (7)
I believe the answer is:
sardine
'fish' is the definition.
(sardine is a kind of fish)
'a diner's chewed' is the wordplay.
'chewed' indicates anagramming the letters.
'a'+'diners'='adiners'
'adiners' is an anagram of 'SARDINE'.
(Other definitions for sardine that I've seen before include "FRENCH FISH" , "Swimmer" , "Small pilchard" , "Young pilchard" , "Rinsed a (anag) - fish" .)