Cold duck I sent back in Spanish snack food (7)
I believe the answer is:
tapioca
'food' is the definition.
(tapioca is a kind of food)
'cold duck i sent back in spanish snack' is the wordplay.
'cold' becomes 'c' (eg on taps).
'duck' becomes 'o'.
'sent back' says the letters should be written in reverse.
'in' indicates putting letters inside.
'spanish snack' becomes 'tapa' (I've seen this before).
'c'+'o'+'i'='coi'
'coi' backwards is 'ioc'.
'ioc' put into 'tapa' is 'TAPIOCA'.
(Other definitions for tapioca that I've seen before include "Cassava product used for puddings" , "it thickens" , "Pudding made from an extract of cassava" , "Starchy pudding" , "Substance from cassava used to thicken puddings" .)