Of fire (7)
I believe the answer is:
igneous
'of fire' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for igneous that I've seen before include "(Rock) set from lava" , "(Of rock) originating in molten state" , "Like fire - type of rock" , "Of fire, fiery" , "Of rocks, volcanic" .)