How do I choose a specific clone closest to a destroyed object?

Hard to know without seeing how you set up your spawn areas and Turrets but if you:

Example

Have sprites as Spawn areas for each turret then you can do:

Turret On Destroyed:

--SubEvent: "Spawn Areas" Pick the closest to (Turret.X, Turret.Y)

----"Spawn Areas" Spawn New Turret

Note:

Don't forget to exclude the "Spawn Area" from Picking of the Turret that you just destroyed