Compound transforming some paint without sulphur (8)
I believe the answer is:
ptomaine
'compound' is the definition.
(ptomaine is a kind of compound)
'transforming some paint without sulphur' is the wordplay.
'transforming' is an anagram indicator (I've seen this in other clues).
'without' indicates named letters should be taken away.
'sulphur' becomes 's' ('S' can be a synonym of 'sulphur').
'some'+'paint'='somepaint'
'somepaint' with 's' taken away is 'omepaint'.
'omepaint' with letters rearranged gives 'PTOMAINE'.
(Other definitions for ptomaine that I've seen before include "Substance found in putrefying matter" , "Sort of poisoning" , "Food poisoning from rotten substances" , "Consequence of decay" , "malodorous substance" .)