Cleaving attack does not work with ranged attacks.
There is however a simple solution to that problem:
Attack Passives System
Probably allows a cleave on ranged attack.
if you want the aoe effect to trigger only when it slows and for it to deal fixed damage just use frost nova with lightning orb(default skill that uses another skill) or something like that.
If what you meant is that you wanted chances to use a ranged attack instead of a melee attack then you probably need a dds(damage detection system) then make a ranged attack effect randomly triggered by attack damage from the unit.
For slowing the attack speed of the unit with the skill(which is something you might have wanted seeing the title of the thread) then you can use the skill granted by gloves of haste but modified to have a negative value.