Possible Clues:

• Poison is poured into the outer one
• It in an ice-cream container is poisonous
• It's poison and an expert on it is brought in
• Poisonous plant, one featuring in a short story
• Plant of the buttercup family
• Poisonous plant producing abnormal reaction right away
• Poison - it's found in an ice cream!
• Flower, for example monkshood or wolfsbane
• To a nice cook, it is poison
• Spring flower - poison