Piece of glass left in part of door (5)
I believe the answer is:
panel
'part of door' is the definition.
(I've seen this before)
'piece of glass left' is the wordplay.
'piece of glass' becomes 'pane' (I've seen this in another clue).
'left' becomes 'l' (common abbreviation).
'pane'+'l'='PANEL'
'in' is the link.
(Other definitions for panel that I've seen before include "Bordered rectangle (in door etc)" , "Committee appointed to judge a competition" , "Quiz team; part of door" , "Group of experts etc" , "Team of judges" .)