Stratum (5)
I believe the answer is:
layer
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">stratum</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for layer that I've seen before include "Source of eggs" , "Early (anag.)" , "One producing eggs" , "(Separate) level" , "Film" .)