Possibly poisonous talk? (3)

I believe the answer is:

gas

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'talk?' is the definition.
(gas can mean to chat)

I don't understand the remainder of the clue.

(Other definitions for gas that I've seen before include "Talk profusely" , "Americans' petrol" , "Something exciting" , "Fumes, vapour" , "Fuel; idle talk" .)

I've seen this clue in the Evening Standard.

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