Of volcanic origin (7)
I believe the answer is:
igneous
'of volcanic origin' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for igneous that I've seen before include "(Rock) set from lava" , "Category of rock, being of solidified molten magma" , "Produced by the action of intense heat" , "produced by fire" , "Formed by fire like volcanic rock" .)