Crossword Genius

Energy of cities, modern and ancient (6)

Ross

I believe the answer is:

vigour

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'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" .)

I've seen this clue in The Times.
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