Filmier oil would ruin coloured glass (10)

I believe the answer is:

millefiori

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'ruin coloured glass' is the definition.
The definition suggests a singular noun which matches the answer.

'filmier oil would' is the wordplay.
'filmier oil' can be anagrammed to 'MILLEFIORI'.
However, I am unsure how this is indicated.

Can you help me to learn more?

I've seen this clue in The Independent.

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