Diners order includes a fish (7)
I believe the answer is:
Here is my best explanation:
'fish' is the definition.
(sardine is a kind of fish)
'diners order includes a' is the wordplay.
'order' indicates anagramming the letters.
'includes' means one lot of letters goes inside another.
'diners' is an anagram of 'srdine'.
'srdine' enclosing 'a' is 'SARDINE'.