Diners order includes a fish (7)
I believe the answer is:
sardine
'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'.
(Other definitions for sardine that I've seen before include "Fish (often tinned)" , "Swimmer" , "Rinsed a (anag) - fish" , "Small fish, usually canned" , "Young pilchard" .)