Glass base graduate left out of church (6)
I believe the answer is:
silica
'glass base' is the definition.
The definition and answer can be both substances as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'graduate left out of church' is the wordplay.
'graduate' becomes 'BA' (someone with a Bachelor of Arts degree).
'left out of' is a deletion indicator.
'church' becomes 'basilica' (basilica is a kind of church).
'basilica' with 'ba' removed is 'SILICA'.
Can you help me to learn more?
(Other definitions for silica that I've seen before include "Compound in quartz" , "Eg, quartz" , "Constituent of sandstone and quartz" , "Oxide occurring as quartz and used in making glass" , "Quartz used in the manufacture of glass" .)