This is a "no prune" Clematis, so shouldn't be cut back. Most plants look dreadful when they finish flowering, its all the effort going into the flowers and not the leaves, so takes them a bit of time to recover and look good again. That said, give it a feed of something like Miracle Gro general purpose, or Growmore stirred into the compost, but, as I'm sure you're aware, this plant will never do so well in a pot, particularly as it matures, as it would in the ground, however large the pot.