To clear the NIMHANS entrance exam for M. Phil in clinical psychology, you need to be thorough with the basics of the syllabus of a PG in psychology either be MA in psychology or MSc in psychology, especially cognitive psychology, biological basis of behavior, abnormal psychology, social psychology research methodology, and all the different branches of psychology from PG level.
My friend is currently pursuing the M.Phil course in clinical psychology at NIMHANS. According to his experience, clearing M.Phil entrance is just one part of the game.
You will have to be passionate enough to take the pressures of having to report to different people, being able to work together with people from different disciplines, and being able to be resilient with minor or major put-offs.
Getting used to the work hours is very exhausting, including Saturdays and at the same time there are no holidays or you cannot leave clinical work while preparing for exams, so the clinical work and patients have the highest priority at this place.
In exchange, you will have the best people guiding you and the day-to-day privilege of working with the best people in the field, so you should be able to manage both academics and clinical work. Not to repeat but you should be mentally strong and constantly enthusiastic about what you are doing and be able to reach out to people when you have difficulties.