I thought this was cool so I uploaded it. Feel free to modify it or anything.


DETAILS:
Superflat world
0 slimes (/killed)

Ok, I_heart_Notch, here are the instructions:

So basically everything's been set up already; the /execute command will be connected to a clock that repeatedly gives out the signal. So assuming you have already INSTALLED the map in your .minecraft file, open the map and check the redstone. IF the clock is working it will spam r-signals to the command blocks. Most probably it don't though since you downloaded it. But no matter; just make another clock connecting to the command block.


So here's to make a simple clock that works in 1.8+:

Key:
l and - is redstone dust
T is redstone torch
^,< and > is repeater
() is command block

Step 1:
-----------
^ l
() l
l l
-----------
Make a rectangle of redstone dust. If you look at the 3rd picture above the command block should be present with a repeater in front.


Step 2: Add more repeaters around the perimeter. 3 should be enough. Set it to 4 ticks (MAX. delay)

Step 3: Here comes the tricky part: Add a power source to the existing circuit like this and then quickly destroy one redstone dust so the torch's power disconnects from the circuit. The red dust(s) should be dug out.


---->------
^ l
() l--------T
l l
-------<---
Now if correctly done so You would have a working clock. If this is killing your eyes please just watch a youtube video.