The term 'It's Straight From The Horse's Mouth' is a phrase that means to get information directly from the source. The crossword solution for this phrase is REIN, which is a four letter word. The term 'REIN' is used to refer to the straps or lines used to control a horse, and thus the phrase 'It's Straight From The Horse's Mouth' is referring to getting information directly from the source, just as one would use reins to control a horse.