Q.What Can I Do About One Of My Green Fennel Tap Root’s Rising Up 3″ (so Far) Out Of The Stable Soil And About To Topple My Plant?
Or snap off at the base. The soil has not been washed away. The plant is leaning over but still anchored firmly in the soil – but only for awhile longer and will not likely survive the upcoming freeze or a taller exposed root. Should I add a soil enclosure built up around this plant? Dig up?

Certified GKH Gardening Expert
This is interesting. I would expect this kind of issue with a lack of light, or with too much shading. This can happen if there are too many taller trees, nearby. I do notice a few Oak leaves in your photo.
In the meantime, these articles will help:
https://www.gardeningknowhow.com/edible/herbs/fennel/growing-fennel.htm
https://www.gardeningknowhow.com/edible/herbs/fennel/fennel-problems.htm
https://www.gardeningknowhow.com/edible/vegetables/fennel-vegetables