Yes, you are eligible to write CLAT. Students passed out from NIOS board can write any professional body exam such as CLAT, Engineering, Medical, etc and every university must accept students from open schooling.  “The Secondary/Sr.Secondary certificate obtained from NIOS have the same recognition as certificates issued by other Boards. NIOS is vested with the authority to conduct Public Examinations at Secondary and Sr. Secondary level vide Govt. of India's resolution dated 14 September 1990. NIOS thus act as another National Board”.     

Yes, students from NIOS boards are eligible to write CLAT exam and all law colleges accept the same. Also, to check the complete eligibility criteria of CLAT you can go through the below link -

CLAT Criteria 2018

Good Luck!!!