Fuel poured over the first glass (5)

I believe the answer is:

flute

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'glass' is the definition.
(flute is a kind of glass)

I don't understand how the remainder of the clue works.

(Other definitions for flute that I've seen before include "See 20" , "Wine glass; instrument" , "Wind instrument" , "Blown" , "Champagne glass" .)

I've seen this clue in The Mirror.

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