Possible Clues:

• Mixture of sodium bicarbonate and cream of tartar
• Local lying in The Sun contacts prisoner-of-war and the German agent responsible for The Rising
• Graduate is on the board with prisoner-of-war and the German agent responsible for The Rising
• Cake raising agent
• Violently break wind outside General Post Office — something in the bread!
• Raising agent
• Culinary substitute for yeast
• WWII fighter pilot, holding king captive, used to make cakes