Egg container upside-down (3)
I believe the answer is:
nit
'egg' is the definition.
(nit is a kind of egg)
'container upside-down' is the wordplay.
'container' becomes 'tin' (tin is a kind of container).
'upside down' says the letters should be written backwards.
'tin' written backwards gives 'NIT'.
(Other definitions for nit that I've seen before include "Louse; fool" , "Eg of louse" , "Foolish person" , "Hair louse" , "Egg; fool" .)