Sheet left on board (5)
I believe the answer is:
panel
'board' is the definition.
(I know that board can be written as panel)
'sheet left on' is the wordplay.
'sheet' becomes 'pane' (pane is a kind of sheet).
'left' becomes 'l' (common abbreviation).
'on' is a charade indicator (letters next to each other).
'pane'+'l'='PANEL'
(Other definitions for panel that I've seen before include "Section of vehicle bodywork" , "Group of interviewers" , "Group selected to judge a competition" , "Jury members" , "Team selected, to judge a competition perhaps" .)