A diner's stewed fish (7)

I believe the answer is:

sardine

'fish' is the definition.
(sardine is a kind of fish)

'a diner's stewed' is the wordplay.
'stewed' indicates an anagram.
'a'+'diners'='adiners'
'adiners' is an anagram of 'SARDINE'.

(Other definitions for sardine that I've seen before include "Stonefish OR? Stone" , "Small pilchard" , "FRENCH FISH" , "Swimmer" , "Young pilchard" .)

I've seen this clue in The Telegraph.

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