Energy displayed in Spanish port and ancient city (6)
I believe the answer is:
vigour
'energy' is the definition.
(I know that energy can be written as vigour)
'spanish port and ancient city' is the wordplay.
'spanish port' becomes 'Vigo' (port in Spain).
'and' means one lot of letters go next to another.
'ancient city' becomes 'ur' (ancient Sumerian city state).
'vigo'+'ur'='VIGOUR'
'displayed in' is the link.
(Other definitions for vigour that I've seen before include "Effort, energy" , "Great strength and energy" , "Potency" , "life" , "Strength, good health" .)