Possible Clues:

• Source of senna
• One who conspired to sacrifice us on a tree?
• Senna-producing tree
• Genus of leguminous shrubs, the pods of which yield senna
• Bark at idiot in US intelligence
• Cold continent including singular source of senna
• Plant carrier inside spy organisation
• This is a blooming silly one with an accountant around
• A blooming fool to be in American Intelligence
• Associate scratched toe by medicinal shrub