In Runescape, by far one of the most profitable and efficient ways of making money, as well as levelling up. The only large downside to this method is that good rewards are not always guaranteed.
If you are a lower combat level, level 1 clue scrolls (easy) will be the best (if not are the best, due to various requirements) you can possibly do. Mid-level players should aim for level 2 (medium) clues and so on. Clue scrolls are recommended to be obtained during slayer tasks, creating a playing environment that is interesting whilst gaining you Slayer and Combat experience, as well as money. Not every slayer monster will drop clue scrolls, but many do.
Players may acquire, as mentioned, low-value rewards from this method, but there is always the chance of items worth 100K+ all the way up to tens of millions; and forhard and elite clues even more. If you have a string of poor rewards, remember you are still gaining valuable XP to gain these rewards. Players constantly killing NPCs for clue scrolls may see profits of millions of GP per day.