Attacking with Fleets
You can attack a fleet belonging to another clan when it is within the highlighted area by right-clicking on it. If the target is further away than your fleet's movement extent will allow, it will take more than one turn to reach it and launch an attack. Once a fleet has reached its target the battle panel will be displayed, prompting you to fight, resolve the battle automatically or withdraw.
Raiding Trade Routes
You can attack another clan's trade by simply selecting your fleet and right-clicking on their trade route. A proportion of their income from that route will be passed to your treasury whilst your fleet remains in place.