Matter comprising sulphur and volcanic rock (5)
I believe the answer is:
stuff
'matter' is the definition.
(stuff is a kind of matter)
'sulphur and volcanic rock' is the wordplay.
'sulphur' becomes 'S' (synonyms).
'and' is a charade indicator (letters next to each other).
'volcanic rock' becomes 'tuff' (I've seen this in another clue).
's'+'tuff'='STUFF'
'comprising' acts as a link.
(Other definitions for stuff that I've seen before include "facts" , "12 [GLUT]" , "Material; eat lots" , "Substance; material" , "set with unspecified ingredients" .)