Gas inert — no good for explosion (8)
I believe the answer is:
nitrogen
'gas' is the definition.
(I know that nitrogen is a type of gas)
'inert no good for explosion' is the wordplay.
'good' becomes 'g' (abbreviation).
'for explosion' is an anagram indicator.
'inert'+'no'+'g'='inertnog'
'inertnog' with letters rearranged gives 'NITROGEN'.
(Other definitions for nitrogen that I've seen before include "It makes up 78% of air" , "Most prevalent gas in air" , "Main constituent of the air" , "Much of what we take in" , "Major component of the atmosphere" .)