Component of sandstone church (but not the northern quarter) (6)
I believe the answer is:
silica
'component of sandstone' is the definition.
The answer and definition can be both substances as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for silica that I've seen before include "Compound in quartz" , "Hard colourless compound from rocks" , "Glassy substance in quartz and sand" , "Constituent of sandstone and quartz" , "Main constituent of sand" .)