Clue: Kidnapper, inexperienced poisoner (5,5)
Answer: PARIS GREEN
Explanation: Definition: poisoner
PARIS (who, depending on your point of view, either kidnapped or eloped with Helen of Troy) + GREEN (inexperienced). Paris Green is a poison used as an insecticide, and, despite its name, a "blue colorant for fireworks" – perhaps our local expert can clarify..