Have left speaking; I don’t react (5)

I believe the answer is:

argon

'i don't react' is the definition.
(noble gases like argon aren't very reactive)

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

(Other definitions for argon that I've seen before include "Chemical element" , "Gas, 1% of atmosphere" , "Organ (anag.)" , "Inert gas used in light bulbs" , "Inert gas in atmosphere" .)

I've seen this clue in The Times.

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