Flammable substance for glass (7)
I believe the answer is:
propane
'flammable' is the definition.
(thesaurus)
'substance for glass' is the wordplay.
'substance' becomes 'pro' (pro is a kind of substance**).
'for' means one lot of letters go next to another.
'glass' becomes 'pane' (I've seen this before).
'pro'+'pane'='PROPANE'
(Other definitions for propane that I've seen before include "Gas used as fuel" , "Type of gas" , "Bottle gas" , "gaseous fuel" , "Bottled fuel gas" .)