Hello Abhishek!!Eligibility criteria to give CLAT examination is mentioned below:
1.A candidate must have qualified Intermediate or equivalent examination of 10+2 education from recognised board.

2.Candidates must have secured at least  45% and for SC/ST 40% is eligible.

So..now you are in class of 11th you can  give clat exam ,when you are appearing for the  12th qualifying examinations and you should have provide an evidence of their passing the qualifying examination at the time of admission.You can apply in  2021 march/april clat examination.Hope the information may helps you.Good luck !!!