Toxic gas over region (5)

I believe the answer is:

ozone

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'toxic gas' is the definition.
'ozone' can be an answer for 'gas' (I've seen this before). I'm unsure of the 'toxic' bit.

'over region' is the wordplay.
'over' becomes 'o' (cricket abbreviation).
'region' becomes 'zone' (zone is a kind of region).
'o'+'zone'='OZONE'

Can you help me to learn more?

(Other definitions for ozone that I've seen before include "Allotropic gas" , "Form of oxygen, O3" , "something found in the atmosphere" , "Pungent form of oxygen" , "Stratospheric layer of gas -- invigorating sea air" .)

I've seen this clue in The Times.

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