Made from lava energy using oxygen to mix (7)
I believe the answer is:
igneous
'made from lava' is the definition.
(igneous rock is made from lava or magma)
'energy using oxygen to mix' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'oxygen' becomes 'O' (atomic symbol for oxygen).
'to mix' indicates anagramming the letters.
'e'+'using'+'o'='eusingo'
'eusingo' with letters rearranged gives 'IGNEOUS'.
(Other definitions for igneous that I've seen before include "Produced by the action of intense heat" , "(Rock) of volcanic origin" , "Originally molten (rock)" , "produced by fire" , "Fire-formed rock" .)