When it reacts saltpetre may be extracted (5)

I believe the answer is:

nitre

I'm a little stuck... Click here to teach me more about this clue!

'be extracted' is the definition.
I can't tell whether this definition defines the answer.

'when it reacts saltpetre' is the wordplay.
'NITRE' is hidden in amongst the letters.
However, I am unsure how the hidden word is indicated.

'may' acts as a link.

Can you help me to learn more?

(Other definitions for nitre that I've seen before include "food preservative" , "found in a slow match?" , "Otherwise saltpetre" , "A part of fertilizer" , "Mineral used in gunpowder" .)

I've seen this clue in The Globe and Mail.

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