As Kumar points out, this has been possible since Sass 3.3.0.rc.1 (Maptastic Maple).

The @at-root directive causes one or more rules to be emitted at the root of the document, rather than being nested beneath their parent selectors.

We can combine the @at-root directive along with interpolation #{} to arrive at the intended outcome.

SASS

.item {
    color: black;
    @at-root {
        a#{&} {
            color:blue;
        }
    }
}

// Can also be written like this.
.item {
    color: black;
    @at-root a#{&} {
        color:blue;
    }
}

Output CSS

.item {
    color: black;
}
a.item {
    color: blue;
}