Six go to ancient city in robust force (6)

I believe the answer is:

vigour

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'robust force' is the definition.
'vigour' can be an answer for 'force' (vigour is a kind of force). I'm not certain of the 'robust' bit.

'six go to ancient city' is the wordplay.
'six' becomes 'VI' (Roman numerals).
'to ancient city' becomes 'ur' (ancient Sumerian city state).
'vi'+'go'+'ur'='VIGOUR'

'in' is the link.

Can you help me to learn more?

(Other definitions for vigour that I've seen before include "Potency" , "Active strength of body or mind" , "Energy, pep" , "Effort, energy" , "Strength, good health" .)

I've seen this clue in The Sun.

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