Upside-down egg container (3)
I believe the answer is:
tin
'container' is the definition.
(tin is a kind of container)
'upside-down egg' is the wordplay.
'upside down' says the letters should be written in reverse.
'egg' becomes 'nit' (nit is a kind of egg).
'nit' back-to-front is 'TIN'.
(Other definitions for tin that I've seen before include "Silver-white element often used in food containers" , "Silver-white metal, used to make cans" , "Kind of metal often used as food container" , "Canister" , "Metal element" .)