I drank the bitter medicine down in one gulp.
It was such a relief to see my father, who looked well.
My great-grandma ate two hundred gyoza in one sitting.
It's weird to think of having one energy. But, what gives you energy? You do it one time every couple of seconds. That would be taking breaths. So, "one energy" is one breath.
The reading is the same as the ones you learned with the individual kanji, though いち from 一 is shortened to いっ.