Clean gas I convert into a pain-killer (9)
I believe the answer is:
analgesic
'a pain-killer' is the definition.
(I've seen this before)
'clean gas i convert' is the wordplay.
'convert' indicates anagramming the letters (I've seen 'converts' mean this).
'clean'+'gas'+'i'='cleangasi'
'cleangasi' anagrammed gives 'ANALGESIC'.
'into' is the link.
(Other definitions for analgesic that I've seen before include "result is soothing" , "number" , "Pain-killer" , "Painkilling" , "Acting to relieve pain" .)