Glass maker may disclaim internal breakages (6)

I believe the answer is:

silica

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'glass' is the definition.
The answer and definition can be both substances as well as being singular nouns.
Perhaps there's an association between them I don't understand?

I can't explain the rest of the clue.

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(Other definitions for silica that I've seen before include "Compound in quartz" , "Constituent of sandstone and quartz" , "Hard colourless compound from rocks" , "Eg, quartz" , "Quartz used in the manufacture of glass" .)

I've seen this clue in the Sydney Morning Herald.

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