Usually rectangular piece of glass left (5)
I believe the answer is:
panel
'usually rectangular piece' is the definition.
'panel' can be an answer for 'piece' (I have seen 'Piece of wood' mean 'panel' so perhaps 'piece' could also mean 'panel'). I'm not certain of the 'usually rectangular' bit.
'glass left' is the wordplay.
'glass' becomes 'pane' (I've seen this before).
'left' becomes 'l' (common abbreviation).
'pane'+'l'='PANEL'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for panel that I've seen before include "Committee to judge a competition or list from which to appoint someone" , "Group of interviewers" , "Section of vehicle bodywork" , "Section of wood or metal" , "List of doctors" .)