Current energy and power source I switched (2,3)
I believe the answer is:
in use
'current' is the definition.
(I've seen this before)
'energy and power source i switched' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'and' is a charade indicator (letters next to each other) (I've seen this in other clues).
'power source' becomes 'sun'.
'switched' says the letters should be written backwards.
'sun'+'i'='suni'
'suni' in reverse letter order is 'inus'.
'e' put after 'inus' is 'IN USE'.
(Other definitions for in use that I've seen before include "Currently employed" , "Being employed - unavailable" , "taken?" , "prevailing" , "Engaged" .)