It means that the person is making progress towards a correct answer. It can also imply that someone has or is improving on a good idea. However, to be "on to something", doesn't need to be to be something that will happen very soon. -The police were really on to something when they suspected that the criminal was still in the area. -Mr. Sandwich was really on to something when he put his meat between two pieces of bread. Now many people use bread to make sandwiches. -I thought I was on to something last night while I was searching for my keys, but I still can't find them. -My brother will turn off his computer and phone at least an hour before he wants to sleep. I think he is really on to something that will help him live a healthier life.

You are right, it does mean that someone is onto a clue or an idea. "...they were onto something that could lead to the location of the murderer. " "they were onto something and that is the murderer could be just around the corner"

If someone is on to something, then they feel they are on the verge of making a discovery.