Mad genius covers Ring of Fire (7)
I believe the answer is:
igneous
'of fire' is the definition.
(I've seen this before)
'mad genius covers ring' is the wordplay.
'mad' indicates an anagram.
'covers' indicates putting letters inside.
'ring' becomes 'O' (letter resembling a ring).
'genius' anagrammed gives 'igneus'.
'igneus' going around 'o' is 'IGNEOUS'.
(Other definitions for igneous that I've seen before include "(Of rock) originating in molten state" , "Of fire, fiery" , "Formed of solidified magma" , "Of fire, like rocks formed from molten material" , "(Of rock) formed from lava or magma" .)