Minecraft wiki entry for Clock, associated text: A lever on, or redstone signal to, any of the four solid blocks can stop the clock. The torch is forced "off" while the dust is lit.
Output can be taken almost anywhere, with a few exceptions:
The blocks "crosswise" from the redstone dust (pistons work, but dust or a repeater is likely to jam the clock).
The block under the repeater (a repeater or piston next to it is out-of-phase, and dust does not light).
Output from the dust side is reverse phase.