Sheet of glass (4)
I believe the answer is:
pane
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">sheet of glass</span>' is the definition.<br/ ><span class="explanation-format__explanation">(pane is a kind of sheet of glass)</span><br/ ><br/ >This is the entire 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" .)