For 28d, take a 6-letter word for flash and remove the first letter ("leader"), leaving you with a 5-letter word for cavalry soldier - a bit unfamiliar this, I thought; the better-known form is the same word with the letter "r" on the end. Anyway, from your 5-letter cavalry soldier the unchecked 3rd or 4th letter needs to be removed. The removal of either would leave a real word, so which one to remove is determined by the comings and goings elsewhere in the grid.