Has headless diner ordered fish? (7)
I believe the answer is:
sardine
'fish?' is the definition.
(sardine is a kind of fish)
'has headless diner ordered' is the wordplay.
'headless' means to remove the first letter.
'ordered' indicates an anagram.
'has' with its initial letter taken away is 'as'.
'as'+'diner'='asdiner'
'asdiner' is an anagram of 'SARDINE'.
(Other definitions for sardine that I've seen before include "Fish in tin?" , "Young pilchard" , "FRENCH FISH" , "Fish (often tinned)" , "A diner's small fish" .)