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