Possible Clues:

• It can be poison
• Tree French novelist climbed
• Tree French author felled?
• Shrub or small tree — Camus (anag)
• Vocally versatile, cruciverbally useful singer Yma who would have turned eighty-seven this week
• Rash-causing shrub
• Mango cousin
• Shrub with a poison variety
• Rash source, maybe
• Total bill for purchase from garden centre?