IME it's easier to get a trellis in before it's needed and you are quite close to needing it. So if it were me, I'd put it in ASAP. It's not hugely difficult to get a clematis to cling later, but it's more fiddly that I want to bother with, so I put them in at first signs of growth or even before. In places where I can, I leave trellises up year round. (Among other things they add winter interest.)

6" is still OK. Just get the trellis in around it or near it.