In this line, the Duke is urging Cesario (Viola in disguise) to take bold action in his pursuit of Olivia. The phrase "be clamorous" suggests making a loud or forceful approach to capture Olivia's attention, and "leap all civil bounds" implies disregarding social norms or restrictions in order to achieve the goal. Therefore, the Duke is encouraging Cesario to be conspicuous and assertive in a way that will ensure Olivia notices him, even if it means disregarding conventional social boundaries.