A mantua was a late 17th century and later rather loose sort of jacket or overdress for a lady, hanging from the shoulders, and open down the front. It could be full-length or less, but usually was worn over a matching robe or dress, as far as I recall. I think in Victorian times the word became sort of mingled with "Mantle" - a similar garment. It seemed in census entries to be a bit more specialised or "up market" than dressmaker.