Forcefulness (6)
I believe the answer is:
energy
'forcefulness' is the definition.
(I know that energy is a type of forcefulness)
This is the entire clue.
(Other definitions for energy that I've seen before include "Capacity for action" , "Healthy capacity for vigorous activity" , "Dynamism" , "Power; capacity for work" , "Vitality, get-up-and-go" .)