Possible Clues:

• African capital
• Capital in vault beneath estate, perhaps, auditors pick up
• City at the confluence of the Blue and White Nile
• Mark turned up with new author that's penned 'Gordon woz 'ere'
• World capital whose name means 'elephant trunk' in Arabic
• The capital of Sudan
• Sudan's capital
• City at the junction of the Blue and White Nile
• African capital where an old banger may end up, do we hear?
• Mark out ground in which to place hospital in capital