I would do harness training (slowly, I think there is a big risk of taking two steps backwards if you try to rush harness training) from when you first get him/her. I am not talking about getting him/her harness trained in a few weeks from when you get him/her, I mean just really slow introduction, perhaps having the lead on your shoulder while your teaching step up, etc. I think it would be much easier if it was just something they recognise as 'normal' from the beginning.
I don't clip, and I think you would probably have better success (long term) if you went through the process of building trust without restricting flight, remember an eclectus will be with you for a long time, so their really is no need to rush things.
I don't think Gizmo was ever clipped and I suspect Pebbles was (or not given the room to fly), and Gizmo has incredibly good flying abilities (flying through partially open doors, going from near floor level to 9 feet high in a very small distance, hovering, etc), whereas Pebbles can fly, but with nowhere near the skill of Gizmo.
I do worry about what if he got out, but my thoughts are, Gizmo stands a much better chance of survival (and then hopefully rescue) if he has great flying abilities. He will hopefully be able to avoid traffic, cats and dogs, predators, etc, get to that tree close by rather than having to fly a greater distance to get the height needed to perch and feel safe (which could possibly be out of sight).
The other reason I often hear is to stop them flying into objects in the house and hurting themselves, but between Gizmo and Pebbles, Pebbles is the one I would be more concerned about loosing altitude or not being able to course correct during a flight.
These are just my personal thoughts obviously, I think clipping or not is a personal choice.
