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Chemical, a possible extract of wintergreen (5)

Ross

I believe the answer is:

nitre

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'chemical' is the definition.
(I've seen this before)

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

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(Other definitions for nitre that I've seen before include "Inter (anag.)" , "A part of fertilizer" , "explosive producer?" , "constituent of gunpowder" , "Chemical" .)

I've seen this clue in the Evening Standard.
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