Clara was trying not to panic and, for the most part, failing. Horribly. Her first reaction had been to reach for her wand, before she remembered that it wasn't working and honestly, that was the only idea she had. What if they couldn't stop the fire? What if the whole common room burned down? What if -
Fortunately Lucius' words snapped her back to reality before her thoughts could grow much more hysterical. "Fireplace - right!" That, at least, made sense. They had needed fire, and here it was (maybe knocking over that candle had been what had done it after all).
Gingerly, and touching as little of the smoldering garment as possible, Clara picked up the coat and tossed it straight into the fireplace.