The capacity for vigorous activity (6)
I believe the answer is:
energy
(Other definitions for energy that I've seen before include "Zest" , "Zeal" , "'Vigour, vitality (6)'" , "Pep" , "Go, zip" .)
I believe the answer is:
energy
(Other definitions for energy that I've seen before include "Zest" , "Zeal" , "'Vigour, vitality (6)'" , "Pep" , "Go, zip" .)