Hello Gautam,

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”.     

Coming to the question, You need to focus on the subject in which you failed. There is a minimum eligibility criterion for CLAT -

• Candidates should have passed Higher Secondary School/Intermediate Examination (10+2) or its equivalent examination with not less than 45% marks in aggregate (40% in case of SC/ST and persons with Disability).
• There is no age bar for appearing in CLAT.

If you qualify these criteria you are eligible to appear in CLAT examination.