Cook overcome by diffused gas (5)
I believe the answer is:
radon
'gas' is the definition.
(I know that radon is a type of inert gas)
'cook overcome by diffused' is the wordplay.
'cook' becomes 'do' (do can mean to cook).
'overcome by' means one lot of letters goes inside another.
'diffused' becomes 'ran' (running is a kind of diffusing).
'do' placed inside 'ran' is 'RADON'.
(Other definitions for radon that I've seen before include "Inert gas that occurs naturally" , "Naturally occurring inert gas" , "Gaseous element, Rn" , "Rare noble gas" , "hazardous substance" .)