can you perplex down someones defense who is covered under defend? or do you have to do it to the clix with defend?

Neither.
If you perplex down a figures defense who is being protected by Defend, they still use the Defending figure defensive value.

Or if you Perplex down a figure with Defend, adjacent figures use the unmodified defensive value. The PAC specifically says unmodified value.

So for example if rookie Invisible Girl is standing next to Mr. Fantastic and you Perplex her defensive value down to 15, Mr Fantastic still has an 18 DV (unmodifed value). Or if you Perplex down Mr.Fantastics DV, he still gets Invisible Girls 18 DV.

The best you could hope for is to Perplex down the Defending figures DV and knock it off of Defend all together.