One with good sense, storing energy of volcanic rock (7)
I believe the answer is:
igneous
'volcanic rock' is the definition.
'igneous' can be an answer for 'rock' (igneous rock is a kind of rock). I'm not certain of the 'volcanic' bit.
'one with good sense storing energy' is the wordplay.
'one' becomes 'i' (Roman numeral).
'with' becomes 'g' (I am not sure about this - if you are sure you should believe this answer much more).
'good sense' becomes 'nous' (nous is a kind of good sense).
'storing' indicates putting letters inside.
'energy' becomes 'e' (physics symbol as in E=mc2).
'nous' placed around 'e' is 'neous'.
'i'+'g'+'neous'='IGNEOUS'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for igneous that I've seen before include "Formed by fire like volcanic rock" , "(Of rock) originating in molten state" , "Fire-formed rock" , "Produced by intense heat, rocks say" , "Of rocks, volcanic" .)