Ouch, Annelise!
My first suspect would be root damage during planting. Did a lot of soil fall off the root ball while planting? Did you here the root ball crack while taking it out of the container?
My second suspect would belack of water. Was the root ball dry when planting? Did you finish the planting with a deep soaking?
My third suspect would be something toxic in the soil, such as some weed killers; oil, paint or thinner; and/or strong fertilizers.

Think a fair bit of soil did fall off Tugbrethil but I didnt hear the root ball crack? Gosh, never heard about things like this affecting the plant! It wasn't dry but it wasnt completely soaking either. Don't think there is anything toxic in the soil. However nearby older Clematis have also have gone like this recently too. What shall I do
with this one ?

I think it's just stress from planting. My advice would be to not touch it at all and next year it will come back fine. I have old clematis too and this time of year they start to shut down. Nothing out of the ordinary.

That looks like clematis wilt to me take a look at the RHS advice

https://www.rhs.org.uk/advice/profile?PID=125

Looks like wilt to me too. A few of mine have been affected with it this year.

Oh dear! Was hoping it wasn't Clematis wilt. Sometimes difficult to tell. Only planted last week... Thanks all.

Assuming yours are all hybrid clematis (not species like montana) it might well be wilt, as the others suggest, though I'm not 100% sure, they're not showing the typical shrivelled, drooping appearance. Just cut out the offending stem at the base, even if that means there's now no growth at all on the plant - it should recover and grow again next year.

Ah! I forgot about Clematis wilt!