Related Clues:

• Two of a century with one of an A in the form of a tree
• After a century or two it turns up A-one as it grows up
• That blooming tree starts with a century at the double
• It takes a double century to start to get growing
• A CA is prickly over the CIA
• How an accountant grows to join American Intelligence
• 'Single C comes at the double, growing with single A (6)'
• An accountant over the blooming detectives
• Whistling thorn, e.g
• Bill joins a security organisation - it has a number of branches
• Wattle one spies under bill
• Current account - one with a network of branches
• Having mechanical sex with accountants, raised plant
• What'll we hear?
• Bill joins a spy organisation as a plant