Hey there

If you want to go for CLAT then ofcourse you need to have good English. For that you'll have to focus on grammar, reading comprehension, vocab also. For grammar you'll have to refer to standard books like rein and Martin etc. For reading comprehension try reading novels and newspapers on a daily basis. Pick difficult words from what you read and then write them in a copy and revise them daily it will boost your vocabulary also.

Good luck