This is a compact, simple, cheap and flexible clock for Minecraft using redstone.
(A clock circuit is a redstone circuit which produces a clock signal: a pattern of pulses which repeats itself.)
Step 1. Materials and first Repeaters.
For this clock, you will need a 3x8 area for the redstone, something to power (in my case, a piston - but it is up to you), a redstone torch, eleven redstone dust, and six repeaters.
First, lay out three repeaters all facing one direction. Make sure they all have zero delay (the two torches on the top of the block should be touching).
Step 2. Second repeaters.
Place the next three repeaters next to the first three FACING THE OPPOSITE DIRECTION (the two torches at the top must be in the opposite direction. You can place these blocks by facing the opposite direction while placing them), with one block's gap between them. They must all face the same (opposite) direction and have zero delay.
Step 3. Connect them.
Link the six repeaters together using ten dust, five on each end with a one block gap like the picture demonstrates.
Step 4. Attach whatever you would like to power.
Use one dust and your item that you would like to power and attach it to one end of the clock.
Step 5. Starting the clock.
Break the redstone in the middle on the far side from where you placed your object to power. Then, break the dust and replace it with a torch. The entire circuit should light up.
Now, pick up your dust and hold it in your hand. Quickly - within a second - break the torch and replace your redstone dust. If you are too fast, the entire circuit will stay powered. If you are too slow, the entire circuit will lose power. If done correctly, your object should power over and over again, and one side of the clock will light up at a time.
Congratulations! You can use this clock for things like traps, music, or parkour in any Minecraft builds or maps you may make.