How to disable clearing color buffer for camera?

We decided to implement our own skybox as a single mesh and a single texture to be rendered in 1 draw call. Unity’s skybox can either render in 6 draw calls using 6 separate textures or in one draw call using cube maps. Unfortunately cube maps don’t compress for mobile devices.

Because the skybox completely covers the screen there is no need to clear the screen before rendering the skybox. How to avoid clearing it in Unity?

You need to set the camera Clear flag to “Skybox” .
You can find it in the hints sections in the Unity documentation here