@JeremyECrawford @mikemearls Can readied attack "when the enemy moves away" be avoided by Disengage?
— Adi Nugroho Tj. (@aditjan90) February 27, 2018
Taking the Disengage action has only one effect: you don't provoke opportunity attacks for the rest of the turn. The Disengage action has no effect on reactions of other sorts. #DnD https://t.co/pXlGBkFigj
— Jeremy Crawford (@JeremyECrawford) February 27, 2018
I seem to recall that the readied action occurs AFTER the catalyst. In this case, after the target leaves the threatened square, you attack. But the attack would miss as the target has already moved away.
So your trigger would be “when my enemy begins to leave my reach.”