I'm not sure I understand correctly if you are saying it is an adjective. The only meaning I could find is perhaps 'candidate' - '. 'I am a candidate in the police entry exams'. Or 'I am competing for a place in the fire service'.

To add to what Gary has said, you can also consider the following since you said 'preparing'. It doesn't meet your criterion for the part of speech you're looking for, but we do use such sentences commonly. In fact, it's more common than saying, for example, 'I am a candidate for _____.' I'm preparing for an exam to be a firefighter. I'm studying for the Scholastic Aptitude Test. I'm appearing for an exam that will get me into the police academy.