Energy of cities, modern and ancient (6)
I believe the answer is:
vigour
'energy' is the definition.
(I know that energy can be written as vigour)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for vigour that I've seen before include "Effort, energy" , "Strength, good health" , "Dynamism - vitality" , "Energy, forcefulness" , "life" .)