Possible Clues:

• It grows back in a first-class way with a couple of accountants
• Tropical tree, inc. mimosa, wattle
• Beginning to amble about, spies a source of gum arabic
• Gum-yielding shrub
• Bill joins a security organisation - it has a number of branches
• After a century a matter of intelligence becomes a thorny matter
• Gum-yielding tree ir shrub
• It's a thorny matter for an accountant to join Intelligence in the States
• How a sound of 21 down gets to an intelligence with thorns
• a wattle.