Store energy transformed as a type of sound (6)
I believe the answer is:
stereo
'as a type of sound' is the definition.
(stereophonic sound)
'store energy transformed' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'transformed' is an anagram indicator.
'store'+'e'='storee'
'storee' with letters rearranged gives 'STEREO'.
(Other definitions for stereo that I've seen before include "Means of amplification of sound through two or more channels" , "Description of record" , "One might play record" , "Matched, rivalled" , "In-depth" .)