Poisonous substance: it's irrelevant, surprisingly (6,7)

I believe the answer is:

silver nitrate

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'poisonous substance' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

'it's irrelevant surprisingly' is the wordplay.
'surprisingly' indicates anagramming the letters.
'its'+'irrelevant'='itsirrelevant'
'itsirrelevant' is an anagram of 'SILVER NITRATE'.

Can you help me to learn more?

(Another definition for silver nitrate that I've seen is " salt used in photography".)

I've seen this clue in The Telegraph.

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