Window left on board (5)

I believe the answer is:

panel

I'm a little stuck... Click here to teach me more about this clue!

This clue might be a double definition.

'window' is the first definition.
(I know that panel is a type of window)

'left on board' is the second definition.
'panel' can be an answer for 'board' (I've seen this before). I'm unsure of the 'left on' bit.

Can you help me to learn more?

(Other definitions for panel that I've seen before include "Board for instruments" , "Rect. section; jury" , "Group of people chosen to judge, discuss etc" , "Dashboard; jury" , "Group of experts etc" .)

I've seen this clue in The Mirror.

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