When this is the case, introduce the idea so that students can see it is simple. Then add complexity incrementally, occasionally reminding students that the original idea was simple. Hashing is fundamentally simple, but it has expanded to the point that it is now reserved until the junior or senior level. A teacher could easily make hashing seem impossible because of its complexity and multiple variations. But the teacher could also introduce hashing by showing that it is extremely simple, and then add complexity incrementally over several class periods.
Some ideas are fundamentally simple, but others are inherently complicated or messy.
Complex concepts require that a number of different techniques and ideas all be understood together before any part of the new concept can be grasped. When trying to present a complicated concept, it is important that students know and understand each sub-concept individually. This can be done by deciding what the important sub-concepts are, and then presenting each one as a separate topic. It is important to tell students you are doing this.