I rushed into the room to cheer them up and clear the air, although I knew it was too late.

clear 是动词，意思是"清除、清理"。 the 是定冠词，特指某个特定的事物。 air 在这里不是指"空气"，而是比喻"气氛、氛围"。 clear the air 是一个固定短语，字面意思是"净化空气"，比喻"消除误会、缓和紧张气氛"。