Egg container up-ended (3)
I believe the answer is:
nit
'egg' is the definition.
(a nit is an insect egg)
'container up-ended' is the wordplay.
'container' becomes 'tin' (tin is a kind of container).
'up-ended' says the letters should be written in reverse (in a down clue, letters go up).
'tin' written backwards gives 'NIT'.
(Other definitions for nit that I've seen before include "Hair louse" , "Stupid person; egg" , "Idiot, ass" , "Young louse" , "Foolish person" .)