To the commenter suggesting using @p and the redstone clock he demonstrated, I have two notes.
First, you are looking for @a to select all players. The selectors are as follows:
@p - selects closest player to the command block
@a - selects all players
@r - selects a random player
@e - selects all entities*
*including hostile/friendly mobs, players, items/blocks on the ground, etc.
it is *strongly* recommended you use modifiers to reduce the scope when
There are far more efficient and faster ways to build clocks, and having fewer repeaters/redstone torches etc is a bonus. The "Redstone Lag" players often experience is actually the result of block light updates, caused by these devices turning on/off. The blocks around them are lit up and dimmed with each cycle, and these lighting changes are what crush some machines.
Pure redstone and redstone blocks tend to have less or no impact on lighting. You can build an incredibly fast clock by simply placing two command blocks against one stone block. In one command block use /setblock to set a redstone block where the stone block currently is. Then, in the other command block, tell it to set the redstone block (using the setblock command) to air (minecraft:air). Lastly, replace the stone block that is adjacent to both command blocks with a redstone block manually. They will instantly start fighting each other for control of the block and an insanely fast, relatively low impact clock is produced.
I would also suggest running /gamerule commandBlockOutput False
otherwise your chat will be filled with line after line of "Block Placed".
For clarity's sake, C is a command block, S is the stone block
Command: /setblock x y z minecraft:redstone_block
C S C
Command: /setblock x y z minecraft:air
both sets of x y z should be replaced with the coordinates for the S block. Use full integers, no decimal places. Alternatively, you can use relative coordinates. These are done as ~ ~ ~ instead of numbers. The three tildes (~) will run the command directly on the command blocks own coordinates. You can modify relative coordinates using positive or negative values. In the drawn example above I could use the following integers. (be sure to know which axis you're adjusting, in this case I've simply used the X axis as an example).
First: /setblock ~1 ~ ~ minecraft:redstoneblock
Second: /setblock ~-1 ~ ~ minecraft:air
This causes the command block to the left to select in the positive direction on the left axis (while maintaining the command blocs y and z). The block to the right does the exact opposite. In this example the only axis that changes is X. The advantage is we can copy this system using a command like clone, or by copying just the code, and not need to adjust the coordinates each time. It saves a lot of work.
Getting the hang of command blocks is a -big- undertaking, an