Very hard to do this sort of thing from a classic clinch, though it is possible to generate a significant amount of force without much windup using a relaxed, "heavy hand" dynamic. My best advice would be to find a realistic base art (that includes a lot of pressure-testing via sparring) and develop your neck strikes as a specialty, in your own time, assuming that it's a serious goal and that the base art doesn't already emphasize these techniques.
that knockout slap is more or less what I am looking for. I have actually been knocked down on krav maga training with this kind of move once. can this slap be successfully applied from a clinch? what kind of art do I need to study for this? krav maga don't spar btw..