Poisoner nicer, as reformed (7)

I believe the answer is:

arsenic

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

'nicer as reformed' is the wordplay.
'reformed' indicates anagramming the letters.
'nicer'+'as'='niceras'
'niceras' anagrammed gives 'ARSENIC'.

Can you help me to learn more?

(Other definitions for arsenic that I've seen before include "Element No 33" , "Powerful poison" , "-- and Old Lace (play)" , "killer" , "Ratsbane" .)

I've seen this clue in The Times.

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