Staunch supporter of glass (4)
I believe the answer is:
stem
This clue might be a double definition.
'staunch' is the first definition.
(synonyms)
'supporter of glass' is the second definition.
'stem' can be an answer for 'supporter' (I've seen this before). I am not sure about the 'of glass' bit.
Can you help me to learn more?
(Other definitions for stem that I've seen before include "Basic skiing turn" , "race" , "Restrain - staunch - supporter" , "Peduncle" , "supporter of flag, for example" .)