NGUI's clipping is done using shaders. NGUI provides shaders that do basic clipping (Unlit - Transparent Colored #, where # is the number of clipped panels). This shader won't combine with the shader used to clip panels via textures (Unlit - Transparent Colored (TextureClip)). They're different approaches to clipping.
In your case the best approach to take would be to create custom render textures for your previews -- you are already using what seem to be render textures, but for some reason you are using a texture-based panel clipping to create the circle around them... why not just create a render texture that already has the circle as a part of it? Then you don't need the extra panels, and the built-in default NGUI shader (Unlit - Transparent Colored) can be used to have everything get clipped nicely.
