Glass key to a vessel (4)
I believe the answer is:
pane
'glass' is the definition.
(thesaurus)
'key to a vessel' is the wordplay.
'key' becomes 'e' (I've seen this in other clues).
'to a' means one lot of letters go next to another (I've seen this in other clues).
'vessel' becomes 'pan' (I have seen 'Shallow vessel ' mean 'pan' so perhaps 'vessel' could also mean 'pan'**).
'e' after 'pan' is 'PANE'.
(Other definitions for pane that I've seen before include "Piece of glass in window" , "Single sheet of glass in a window" , "Panel of glass" .)