Bit of food from my tin sent back (5)
I believe the answer is:
nacho
'bit of food' is the definition.
(nacho is a type of snack)
'my tin sent back' is the wordplay.
'my' becomes 'oh' (both are expressions of surprise).
'tin' becomes 'can' (synonyms).
'sent back' says the letters should be written backwards.
'oh'+'can'='ohcan'
'ohcan' written backwards gives 'NACHO'.
'from' is the link.
(Other definitions for nacho that I've seen before include "Mexican corn chip eaten with cheese, etc" , "Small piece of tortilla with a topping" , "chip production line unit" , "kind of chip" , "Mexican bite" .)