Glassy substance in quartz and sand (6)
I believe the answer is:
silica
'glassy substance in quartz and sand' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for silica that I've seen before include "Constituent of sandstone and quartz" , "Mineral found as quartz and opal" , "Quartz used in the manufacture of glass" , "Hard colourless compound from rocks" , "Main constituent of sand" .)