I have a backlog in my B.Com and I dont want to waste a year. What shall I do to pursue MBA in finance?

To appear for CAT, you need to first complete your graduation with minimum 50% marks at graduation level. Since you have a back log you can use your time to enrol yourself for a certification course in finance related subjects or also prepare for CAT by joining a coaching institute & simultaneously

Without clearing the backlog, you cannot get admission in the higher degree course. For MBA, you need to have minimum 50% marks in Bachelors and need to clear CAT with good scores.