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Toxic gas initially in borders of Colorado (6,8)

Ross

I believe the answer is:

carbon monoxide

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'toxic gas' is the definition.
(I've seen this before)

I don't understand how the remainder of the clue works.

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(Other definitions for carbon monoxide that I've seen before include "CO" , "Toxic gas" , "inspired to kill" , "POISON" .)

I've seen this clue in The Daily Mail.
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