What is the best way to describe Clay Lee's relationship with her father in Almost Innocent?
This is an interesting question in that Clay-Lee is actually not the protagonist of the novel; her mother, Constance, is. In my opinion, the relation between the two could be best described as the relation between two people who are "almost innocent" equals and are, almost, of the same maturity level.
It is commonly known within the novel that Rand (as well as his wife, Constance) are both childish. In fact, when explaining the name Mishka (name of the dog, the sailboat, and the still-born child), Rand says something interesting.
[Mishka reminded Constance of what is] rich and simple and impossible to find again of her life before she married me.
Rand continually confides in Clay-Lee throughout the novel. Most importantly, Clay-Lee knows that Constance blamed Rand for the death of the baby. Of course, it is Clay-Lee who ignores her mother's cries of suffering during the birth, which is the real reason why both Constance and the baby die. Because Clay-Lee repents of this sin, she is considered "almost innocent."
There is even more about the father/daughter relationship in the epilogue. Clay-Lee is now a woman and watches, helpless, as Rand drinks alcohol to his death. As his equal, Clay-Lee, even as a grown woman, protects her dad from the real truth: the child that died wasn't really Rand's; it was the child of Uncle Baby Brother.