Board left under piece of glass (5)
I believe the answer is:
panel
'board' is the definition.
(I know that board can be written as panel)
'left under piece of glass' is the wordplay.
'left' becomes 'l' (common abbreviation).
'under' says to put letters next to each other (below, in a down clue).
'piece of glass' becomes 'pane' (I've seen this in another clue).
'l' after 'pane' is 'PANEL'.
(Other definitions for panel that I've seen before include "Rect. section; jury" , "Piece of wood" , "Team of judges" , "Body section" , "- Game; - saw; - pin" .)