Container in which egg is sent back (3)
I believe the answer is:
tin
'container' is the definition.
(tin is a kind of container)
'egg is sent back' is the wordplay.
'egg' becomes 'nit' (nit is a kind of egg).
'is sent back' is a reversal indicator.
'nit' reversed gives 'TIN'.
'in which' acts as a link.
(Other definitions for tin that I've seen before include "Silvery-white metallic element" , "Silver-white metal, used to make cans" , "Cornish metal?" , "Ready" , "A silvery-white metal often in cans" .)