If you are sure that your clematis is getting enough water, then this could be a case of magnesium deficiency. Some tomato fertilizers contain magnesium since tomatoes also need this trace element. You could also try the following remedy: Mix one teaspoon of epsom salt to one gallon of water. The mix can be applied in quart amounts at the base of the plant over a period of time until the yellow goes away. I hope this works for you.

thanks, Ive been trying the epsom salt idea for a time, I have got 10 clematis diff var. around the garden all in containers, 9 seem to be behaving, 1 Is'nt, it flowered very well but now seems dying, the stems are going black and brittle I am wondering if there is to much growing medium in the pot for the roots to accept, might try trimming right down, lifting the pot into the cold greenhouse, let it dry right out, and see if the roots will send out new shoots. Lammie (Roy).

Is the plant collapsing? Could it be Clematis wilt? In which case, then do cut it hard back and hope. It shouldn't be the amount of growing medium. They like being deeply planted.

Thanks, will try cutting back hard, and see what next year brings