A poison (7)

I believe the answer is:

cyanide

'a poison' is the definition.
(cyanide is poisonous)

This is the entire clue.

(Other definitions for cyanide that I've seen before include "Nice day (anag) - poison" , "Highly toxic substance" , "Poisonous compound" , "killer" , "Extremely poisonous salt" .)

I've seen this clue in the Sydney Morning Herald and The Telegraph.

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