The animation for Clean Sweep shows the opposing card and it's adjacent cards getting cut with a horizontal slash. The opposite card takes damage first, then the one on the left then on the right.
Description[]
When a card with Clean Sweep deals physical damage, the cards adjacent to the opponent card take the same damage.
If your Clean Sweep card also possesses abilities like Weaken, only the card directly targetted will be affected by the ability, but it will be affected multiple times. Ex: If the Tiamat in the picture had Weaken, the Lion-Lady would be affected by Weaken 3 times; once for itself, once for the Bone Dragon, and once for the Corpse Guard.
If the defending card successfully dodges the attack no damage will be dealt to the adjacent cards.
If the damage is reduced, due to abilities like Parry or Ice Shield, the amount of damage the adjacent cards receive is whatever the reduced amount that the main card received. This damage can further be reduced if the adjacent cards also possess Parry or Ice Shield.