what if I couldn't clear cat but I want to master in the field of finance. If not the top colleges then what way should I choose

CAT is not an exam like IIT-JEE where you get selected or not selected. CAT exam by default help you in applying in IIMs. But thats not the end for the CAT exam marks. Through this exam you can apply individually to many private colleges. Then according to your percentile and the students who have applied in the college you will be called for next round of interview. Some of the colleges even enroll you directly, process of each college need not to be same. It is according to their norms and policies.

So by appearing in CAT you can get into many colleges. Other than CAT exam there is XAT, SNAP, NMAT, IIFT, MAT, CET, CMAT exams whose scores are accepted by many colleges. So you can appear in these exams also which can help you in going to a college where you can do masters in finance.