Eric's an unusual type of poisoner (7)

I believe the answer is:

arsenic

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'type of poisoner' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

'eric's an unusual' is the wordplay.
'unusual' is an anagram indicator.
'erics'+'an'='ericsan'
'ericsan' anagrammed gives 'ARSENIC'.

Can you help me to learn more?

(Other definitions for arsenic that I've seen before include "Poisons in scare" , "Ratsbane" , "Element with many poisonous compounds" , "Brittle grey-green semi-metal, atomic number 33" , "Poison used in pesticides and weedkillers" .)

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

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