I want more energy and new heart (6)
I believe the answer is:
encore
'i want more' is the definition.
(call for more from a performer)
'energy and new heart' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mco).
'and' is a charade indicator (letters next to each other).
'new' becomes 'n' (abbreviation as in New Testament NT).
'heart' becomes 'core' (synonyms).
'e'+'n'+'core'='ENCORE'
(Other definitions for encore that I've seen before include "Cry for repeat performance" , "Bonus" , "Enthusiastic call" , "Play or sing it again" , "extra performance" .)