Each answer describes
an UNDERGROUND LINE on the London tube. Also clued by the title: clandestine = underground and contour = line
D, in (pin -p), Washington DC : the D in Washington DC stands for DISTRICT
New York, Opera, Venue (avenue - A) : the METROPOLITAN is an opera house in NYC
National (nationals -s), Captial, Seychelles : the capital city of Seychelles is VICTORIA
Belgium, Battle, Site : WATERLOO is a famous battle site in Belgium
Credit to the other two answers 1 and 2 which both gave partial answers that helped me along the way.