thunderclap work on wep mods?

if i cast thunderclap on an enemy and i have shocking mod on daggers, everytime i hit them will they get KD?

Correct!

Be careful though, because every time that enemy is hit by you (or anyone else with lightning damage), you will lose the energy.

yes
This works best with a short bow. Knockdown is 2 seconds, so you should have a weapon with a refire of at least 2 seconds. Shortbow being the closest one to those 2 seconds.

If you plan to use it on your assassin, consider [skill]Shock[/skill]Instead

kool ty, was just wondering if it would be worthwhile to use for constant KD if i have high energy to be able to still use other skills.

A shortbow is the best thing to use if you're going to try this. If you use daggers, you'll hit them while they're already, so they won't be knocked down, but you'll still lose energy. If that didn't make sense, remember to not hit them while they're already knocked down.

Thunderclap is fun fun fun to play but its not really practical for anything.

Shortbow + right string is all you need.