The capital of Portugal (6)
I believe the answer is:
lisbon
'the capital of portugal' is the definition.
('Lisbon' can be a synonym of 'capital of portugal')
This is all the clue.
(Other definitions for lisbon that I've seen before include "Capital of Portugal" , "Capital city and Treaty" , "Capital city on the Tagus" , "Portuguese capital" , "European capital" .)