Drano will work to clear a clog if there's just a little standing water but if there's too much water then the water will dilute the Drano too much to allow it to work like it should.
The more strength and less diluted the Drano has or is then the better the Drano will work to clear the clog but if the Drano is diluted too much with too much water then it loses it's strength and won't work as great.
Try removing as much standing water that you can from the drain and then pour in the Drano and allow it to sit for 30 minutes to an hour to allow it to work on clearing the clog.