Energy, vigour (3)
I believe the answer is:
vim
I believe this clue is a double definition.
'energy' is the first definition.
(I know that energy can be written as vim)
'vigour' is the second definition.
(I know that vigour can be written as vim)
(Other definitions for vim that I've seen before include "Energy and vigour" , "Pizazz" , "Pep (informal)" , "Informal energy or drive" , "Pep and vigour" .)