Inner layer (6)
I believe the answer is:
lining
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">inner layer</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for lining that I've seen before include "See 21" , "Inner covering" , "waistcoat material" , "Layer on the inner surface" , "Inlay" .)