Keep energy loud (5)
I believe the answer is:
forte
'loud' is the definition.
(I know that loud can be written as forte)
'keep energy' is the wordplay.
'keep' becomes 'fort' (I've seen this before).
'energy' becomes 'e' (physics symbol as in E=mc2).
'fort'+'e'='FORTE'
(Other definitions for forte that I've seen before include "sort of musical passage" , "Loud (mus.)" , "Skill" , "Thing at which one excels" , "Musically, loud" .)