I don't recommend trying to apply a clear coat over the paint. The effort needed to apply a clear is the same as painting. With that much effort you should repaint. Besides, clear coats also oxidize, sometimes faster that paint.
Oxidation is the way that paint deals with UV light. The best way to prevent excessive paint oxidation is to use a high quality 100% acrylic house paint with a good sheen, satin or semi-gloss. Acrylic paints are more resistant to UV light when compared to oil based paints.
Also, the thickness of the paint film is important. I always apply 2 coats of paint. The first coat protects the house and the second coat is the one that oxidizes. This way the maximum life span for the paint job is achieved.
Right now, the best way to deal with your oxidizing paint is an occasional washing with a mild environmentally friendly soap and water. This can be once a year with low pressure from a power washer. Keep an eye on the paint film. It thins as it oxidizes and can form cracks when too thin. These cracks will eventually peel.