A poisonous sort of “ology” (5)

I believe the answer is:

toxic

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'a poisonous' is the definition.
(I've seen this before)

I cannot understand how the rest of the clue works.

(Other definitions for toxic that I've seen before include "word describing bad things" , "Deadly - venomous" , "Poisonous, noxious" , "Harmful - cox it (anag)" .)

I've seen this clue in the Evening Standard.

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