Help me eliminate jagged edges please (LR5)

[Janesy B]

I seem to be struggling with jagged edges on some shots, even with a tiny bit of sharpening and output sharpening turned off. Is it just down to the fact that it's a large image resized down?

100% crop

Predictably with Flickr's OTT sharpening it looks a mess on there..

Ford Shelby Mustang GT350 by Bryan Janes, on Flickr

Any tips would be appreciated.