Surprisingly clean yet ultimately effective gas (9)
I believe the answer is:
acetylene
'gas' is the definition.
(acetylene is a gas)
'surprisingly clean yet ultimately effective' is the wordplay.
'surprisingly' is an anagram indicator (a surprising ordering of the letters).
'ultimately' indicates one should take the final letters.
The final letter of 'effective' is 'e'.
'clean'+'yet'='cleanyet'
'cleanyet' anagrammed gives 'acetylen'.
'acetylen'+'e'='ACETYLENE'
(Other definitions for acetylene that I've seen before include "Flammable gas" , "Illuminating stuff also used for joining" , "Gas used chiefly in welding" , "cutting gas emission?" , "powers industrial torch" .)