I was. I added a lamp after the comparator to make sure it wasn't working
Three command blocks in an arrow formation facing right or left (http://gyazo.com/d7b86faf2b1e7eed837bc2a0270504bb)
Top block: setblock ~ ~-1 ~ minecraft:air
Right block: /execute @p ~ ~-1 ~ fill ~ ~ ~ ~ ~ ~ slime 0 replace emerald_block
Bottom block: setblock ~ ~1 ~ minecraft:redstone_block 0 destroy
Comparator from right block, redstone block between top and bottom (creates a fast clock)
My mistake, had another bug in here but removed and out in a different thread.. Forgot to update title