Liveliness and energy coming to old city after six (6)
I believe the answer is:
vigour
'liveliness' is the definition.
(vigour is a kind of liveliness)
'energy coming to old city after six' is the wordplay.
'energy' becomes 'go' (I've seen this in other clues**).
'coming to' means one lot of letters go next to another.
'old city' becomes 'ur' (ancient city state).
'after' is a charade indicator (letters next to each other).
'six' becomes 'vi' (roman numeral).
'go'+'ur' is 'gour'.
'gour' after 'vi' is 'VIGOUR'.
'and' acts as a link.
(Other definitions for vigour that I've seen before include "Physical strength and health" , "Active strength of body or mind" , "Potency" , "Force" , "Energy, dynamism" .)