See-through item (4)
I believe the answer is:
pane
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">see-through item</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for pane that I've seen before include "Panel of glass" , "Piece of glass in window" , "Single sheet of glass in a window" .)