'Seat of Government': article by ex-party leader (3,5)
I believe the answer is:
the hague
'seat of government' is the definition.
(seat of the government of the Netherlands)
'article by ex-party leader' is the wordplay.
'article' becomes 'the' (the definite article in English).
'by' says to put letters next to each other.
'ex-party leader' becomes 'Hague' (William Hague, former Conservative Party leader).
'the'+'hague'='THE HAGUE'
(Other definitions for the hague that I've seen before include "Seat of government of the Netherlands" , "Dutch seat of government" , "Netherlands city" , "'Seat of the International Court of Justice (3,5)'" , "Location of the International Court of Justice" .)