Vigour, vitality (6)
I believe the answer is:
I believe this clue is a double definition.
'vigour' is the first definition.
(I know that vigour can be written as energy)
'vitality' is the second definition.
(I know that vitality can be written as energy)
(Other definitions for energy that I've seen before include "Vivacity" , "Gusto" , "Zeal" , "Vigour, power" , "Strength, vitality" .)