Strength, vitality (6)
I believe the answer is:
energy
I believe this clue is a double definition.
'strength' is the first definition.
(energy is a kind of strength)
'vitality' is the second definition.
(I know that vitality can be written as energy)
(Other definitions for energy that I've seen before include "Seventeen forty six defeat for Bonnie Prince Charlie" , "Verve" , "Dynamism" , "Pep" , "Vitality, power" .)