Anti-anti-gas-explosion? (7)
I believe the answer is:
against
'anti' is the definition.
(I've seen this before)
'anti-gas-explosion?' is the wordplay.
'explosion?' is an anagram indicator (I've seen 'explosion' mean this).
'anti'+'gas'='antigas'
'antigas' is an anagram of 'AGAINST'.
(Other definitions for against that I've seen before include "Opposite to" , "In competition with" , "on the other side" , "Versus - in contact with" , "In opposition to" .)