You can start the preparation for the CLAT UG in Class 11. However, you cannot appear for the CLAT exam in Class 11. According to the CLAT UG eligibility criteria, candidates who have appeared for the Class 12 examination or are appearing in the Class 12 examination are eligible to apply for the entrance test. Candidates must have passed the Class 12 or an equivalent examination with a minimum of 45% marks or its equivalent grade (40% marks or equivalent grade for the SC/ ST/ PwD category candidates). Therefore, you must have passed the Class 12 examination in order to appear for the CLAT UG and cannot appear for this entrance test if you are in Class 11.