A hormone is a chemical substance produced in one part of an organism that moves to another part to bring about a specific effect, regulated by a feedback mechanism.
Source: What you have learnt, Chapter 6
The examiner expects a concise definition covering two key points: (1) hormones are chemicals produced in one part and (2) they act on another part. Mentioning the feedback mechanism adds completeness. Avoid long explanations for a 1-mark question.