Gas is clean yet unfortunately emetic initially (9)
I believe the answer is:
acetylene
'gas' is the definition.
(acetylene is a gas)
'clean yet unfortunately emetic initially' is the wordplay.
'unfortunately' is an anagram indicator (a bad or unfortunate spelling).
'initially' suggests taking the first letters.
The initial letter of 'emetic' is 'e'.
'clean'+'yet'='cleanyet'
'cleanyet' anagrammed gives 'acetylen'.
'acetylen'+'e'='ACETYLENE'
'is' is the link.
(Other definitions for acetylene that I've seen before include "Illuminating stuff also used for joining" , "Gas used chiefly in welding" , "cutting gas emission?" , "powers industrial torch" , "Flammable gas" .)