newbie here, I have made elderflower champagne and elderflower cordial this year. I assume elderflower cider is apple cider flavoured with elderflower? Elderflower is lemony and fresh and sort of perfumey, I would make cordial from an old fashioned recipe and add it to the brew at, say, half the concentration that you would add it to soda water or lemonade. I am guessing though.