Stratum or seam (5)
I believe the answer is:
layer
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">stratum</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >'<span class="explanation-format__original">seam</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(Other definitions for layer that I've seen before include "Single thickness, tier" , "Single thickness and hen" , "Coating" , "Early (anag.)" , "course" .)