Food a large number unfortunately sent back (5)
I believe the answer is:
salad
'food' is the definition.
(salad is a kind of food)
'a large number unfortunately sent back' is the wordplay.
'a large number' becomes 'd' ().
'unfortunately' becomes 'alas' ('alas' is used to express regret).
'sent back' says the letters should be written backwards.
'd'+'alas'='dalas'
'dalas' written backwards gives 'SALAD'.
(Other definitions for salad that I've seen before include "Raw vegetable dish" , "Raw, green, veg" , "Lettuce, cucumber etc" , "One may well be green" , "Cold dish of various vegetables," .)