It's measured in joules or ergs (6)
I believe the answer is:
energy
'it's measured in joules or ergs' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for energy that I've seen before include "Vitality, get-up-and-go" , "Vigour, vim, vitality" , "Capacity for action" , "Gusto" , "Power - drive" .)