A Clearing wave is the 1st (Or 2nd if you count Scouting) wave of attacks launched on a city. It is the brunt of the attack force, and is used to destory the enemy units, making them a sitting duck (or sitting fortress). As for how many units should be in the clearing wave, it depends. You should have mainly Berserkers, Guardians, and Rams. You should be aware that the enemy has most likley used Attack Dodging, and that regardless of your scouting, you should expect (or be prepared) for a Full City of Troops. In Conclusion, make sure that you time your Clearing wave to hit Right before your Scholar Wave(s). Since Scholars are the slowest unit, you will ironically send out your Scholars 1st, and send your Clearing wave so it is AOT (Arriving on Target) just seconds before the Scholars.