"Clear-cut" in this context means "restricted - well defined, (in the sense of) limited within boundaries." You might say the edges are clean, and not fuzzy, figuratively speaking.
"Clear" means "not confused," or something like that.
If I'm not mistaken, "clear-cut" is similar to saying that something's very obvious or easy to understand. I actually haven't head that saying in a while. "Clear" can mean the same thing, but it can also mean something that is translucent or unblemished, such as a clear glass, or clear skin.
so actually both clear and clear-cut can have exactly the same meaning, i.e. if something is clear, or clear-cut, that means it's "easy to understand" or "presented in a way that is easy to understand" ?
The difference between a fetus and a person is not clear-cut. (We don't know exactly where to draw the boundary.) You could substitute "clear" here, and the meaning would be almost the same.
The difference between a V-6 engine and an inline 6 engine is clear-cut. (You could substitute "clear