I screwed up and grossly overcooked some of the parameters of an otherwise quite good Clarity tool in Topaz.
The problem with many clarity/structure tools is that when increasing localized contrast they create halos like those around the trees. Topaz is actually quite good at suppressing them but nothing is foolproof.
It's far superior to using Clarity in Light Room/ACR or using a large radius, low amount sharpening and is similar to Structure in The NIK collection/Snapseed.

In this instance I was more interested in the sky and used ReMask in the opposite way I normally would, that being to cut out the trees and ground and layer them over a sky.
In this case I used mask to cut out the sky...darken it, use Clarity...and composite it back down.