报晓 is a literary verb-object compound meaning 'to announce the dawn', typically used for roosters crowing at daybreak. It is a separable verb (离合词): the object 晓 can be separated from 报 by a measure word or duration, e.g., 报了一声晓 ('let out a dawn-crowing sound'). You wouldn't use it with a direct object; instead, the time or context comes before the verb.