For a single chain lightning when Thunder Clap is cast, create 1 dummy when you cast Thunder Clap, add an expiration timer worth 5 seconds or so, order it to cast on an affected unit.
For a chain lightning on every affected unit, you'll need to use the action you'll have to follow the actions I'm listing below:
For each (Integer A) from (Number of units in (Units within 512.00 of (Position of (Casting unit)) matching ((((Matching unit) belongs to an ally of (Owner of (Casting unit))) Equal to False) and (((Matching unit) is alive) Equal to True)))) to (Number of units in (Units within 512.00 of (Position of (Casting unit)) matching ((((Matching unit) belongs to an ally of (Owner of (Casting unit))) Equal to False) and (((Matching unit) is alive) Equal to True)))), do (Actions)
Loop - Actions
Unit - Create 1 Dummy for (Owner of (Casting unit)) at (Position of (Casting unit)) facing Default building facing degrees
Unit - Add a 5.00 second Generic expiration timer to (Last created unit)
Unit - Order (Last created unit) to Orc Far Seer - Chain Lightning (Random unit from (Units within 512.00 of (Position of (Casting unit)) matching ((((Triggering unit) is alive) Equal to True) and (((Triggering unit) belongs to an ally of (Owner of (Casting unit))) Equal to False))))
There's another way to do it, but requires you to use a negative valued Healing Wave instead of Chain Lightning. That version allows you to send one chain lightning to each affected unit, but is harder to customize, and is very tedious to adjust the dummy abilities to match with the Hero skill level. I'd post it, but erm... it's tedious... and it's 9:22 here. I'm sleepy. I'll give it to you if you're unsatisfied with this trigger some other time. I'm also checking the other version for bugs, particularly wether it's MUI... Gudluk with whatever you're using this spell for, dude.