Tier, stratum (5)
I believe the answer is:
layer
This clue is a double definition.
'tier' is the first definition.
(I've seen this before)
'stratum' is the second definition.
(I know that stratum can be written as layer)
(Other definitions for layer that I've seen before include "One supplying eggs" , "Stratum; chicken" , "Lamina" , "Film" , "Early (anag.)" .)