In Illustrator, it's not currently possible to use Select > Next Object Above / Below when working in isolation mode, e.g. artwork inside a clipping mask.
I can still open the layers palette and see the stacking order in the isolation mode, and select artwork there that is otherwise tricky to select. This is a bit cumbersome though.
Does anyone have a quick way to select the next object above / below while in isolation mode?
I have made a feature request for this functionality.