Possible Clues:

• After a century or two it turns up A-one as it grows up
• It's a thorny matter for an accountant to join Intelligence in the States
• Two of a century with one of an A in the form of a tree
• Gum-yielding tree ir shrub
• Garden plant also known as wattle
• Australian wattle
• Locust tree, e.g.
• Twice with a hundred and one A grows up
• Tree (wattle or robinia)
• Plant an accountant in a federal US bureau