In plain words
Then, seeing Dhritarashtra's sons arrayed and as the clash of weapons was about to begin, the son of Pandu whose banner bore a monkey raised his bow—
Word for word
Then, having seen the sons of Dhṛtarāṣṭra standing in array, he whose banner bears the monkey, as the clash of weapons was beginning, having raised [his] bow, the son of Pāṇḍu