CLAT

It is not necessary at all. CLAT is just to take admission in a Law Degree/Law College. To appear in the civil services or you say IAS you need to be a graduate in any stream.

You need to score around 60 to 70% of marks in class 12th boards. Best of luck. You need to score around 150 to 160 in CLAT to get a good law college. The choice is yours.

CLAT score does not ensure concession in fee. In case any particular college has the same, they would have their own criteria for the same.

Pattern of Exam
The CLAT is an objective type examination of 200 marks. Each question carries one mark. The duration of the examination is of two hours. Out of the 200 marks, the subject areas with weightage are as the following:
English including comprehension - 40 marks;
General knowledge and current affairs - 50 marks;
Elementary mathematics - 20 marks;
Legal aptitude - 50 marks
Logical reasoning - 40 marks.

You could also go for:
Universal Institute of Legal Studies (UILS)
It has its centers at Connaught Place and Mukherjee Nagar as well.
It is also a good institute apart from CLAT possible and Career Launcher.

Yes, if your aim is only law and the best law college, you should drop a year. However, it's not compulsory, if you are intelligent much and prepared for CLAT, you don't need to do this.
All the best for CLAT.

Candidates should have passed Higher Secondary School/Intermediate Examination (10+2) or its equivalent examination with not less than 50% marks in aggregate (45% in case of SC/ST/OBC and persons with Disability). Candidates appearing in the qualifying examination can also appear provisionally.

Definitely, you can since the age bar has been lifted from CLAT in 2016.
You need to have 60% in 10+2 boards or equivalent, at least.
Good luck!

Online Form Submission: 01st June 2017 to 15th June 2017
Application Fees Payment (through SBI Collect): 01st June to 16th June 2017
Online Admit Card Download: 21st June to 24th June 2017
Entrance Test: 25th June 2017 (Sunday)
Entrance Test Overall Merit List & Category wise seat Capacity: 30th June 2017
These were the dates which are over for the current year.
For 2018 you need to wait till Jan/Feb for the dates to be finalized.