parents.scss

// SASS
nav {
  a {
    color: green;
    li.selected & {
      color: red;
    }
  }
}

// OUTPUT I WANT
nav a { color: green; }
nav li.selected a { color: red; }

// OUTPUT I GET
nav a { color: green; }
li.selected nav a { color: red; }

Don't nest for the sake of nesting ;)

nav {
    a {color: green}
    li.selected a {color: red;}
}