A politician initially dismissed methane (8)
I believe the answer is:
firedamp
'methane' is the definition.
(I've seen this before)
'a politician initially dismissed' is the wordplay.
'politician' becomes 'MP' (Member of Parliament).
'initially' means one lot of letters go next to another.
'dismissed' becomes 'fired' (synonyms).
'a'+'mp'='amp'
'amp' after 'fired' is 'FIREDAMP'.
(Other definitions for firedamp that I've seen before include "Coal mine peril" , "Gas lit by a member" , "Something potentially dangerous in mines" , "a particular horror of mine" , "dangerously explosive" .)