Here's a breakdown:
* Lean: This can be a noun, meaning "a slope or inclination."
* Example: "The road had a sharp lean to the right."
* Leanness: This is the noun form emphasizing the state of being thin or slender.
* Example: "The leanness of her figure was striking."
The choice between the two depends on the context. If you're talking about a physical characteristic, "leanness" is more appropriate. If you're referring to a slope or inclination, "lean" is the correct noun.