The capital of Lebanon (6)
I believe the answer is:
beirut
'the capital of lebanon' is the definition.
('Beirut' can be a synonym of 'capital of lebanon')
This is all the clue.
(Other definitions for beirut that I've seen before include "Middle East capital" , "Capital of 21 across" , "Capital city of Lebanon" , "Lebanon's capital" , "Lebanese capital" .)