Reflection anonymous struct field pointer

I have a struct like this

type duration struct {
    time.Duration
}

and another one like that

type Config struct {
    Announce duration
}

I am using reflection to assign flags to the fields of the struct Config. However, with the particular use case of the type duration, i am stuck. The problem is that when i do a switch type, i got *config.duration instead of *time.Duration. How can i access the anonymous field ?

Here is the full code

func assignFlags(v interface{}) {

    // Dereference into an adressable value
    xv := reflect.ValueOf(v).Elem()
    xt := xv.Type()

    for i := 0; i < xt.NumField(); i++ {
        f := xt.Field(i)

        // Get tags for this field
        name := f.Tag.Get("long")
        short := f.Tag.Get("short")
        usage := f.Tag.Get("usage")

        addr := xv.Field(i).Addr().Interface()

        // Assign field to a flag
        switch ptr := addr.(type) { // i get `*config.duration` here
        case *time.Duration:
            if len(short) > 0 {
                // note that this is not flag, but pflag library. The type of the first argument muste be `*time.Duration`
                flag.DurationVarP(ptr, name, short, 0, usage)
            } else {
                flag.DurationVar(ptr, name, 0, usage)
            }
        }
    }
}

Thanks