This clock works with one observer, 7 amplifiers and a bit of redstone dust.

 

This clock works with one observer, 8 amplifiers and a bit of redstone dust.

If it is activated, it takes a short time, then it first flashes and turns on permanently after 28 ticks, after it is switched off it takes 29 ticks.

The clock is very compact, so it can be built over a ceiling etc..

Instructions:

I think it's all recognizable in the picture. Otherwise write to me: gamer07_2@web.de