Egg containers in the plainest sense (5)
I believe the answer is:
nests
'egg containers' is the definition.
(I've seen this in another clue)
'in the plainest sense' is the wordplay.
'in' indicates the answer is hidden within the clue.
'NESTS' is hidden in the letters of 'plainest sense'.
(Other definitions for nests that I've seen before include "Settles comfortably into, as birds do" , "Structures for laying eggs" , "Eyries" , "Sets of small tables that fit closely together" , "Structures for egg-laying" .)