Clems do this sometimes. Just cut down the bad parts and should come up again. You don't need to do anything to the soil. This wilt cannot be treated by chemicals. Generally, they outgrow it as they get older.

My 5 year old Sunset turned brown this summer for the first time ever. I cut it back to about 18" from the ground & watered it a little more. It has come back to green foliage. I just call it a freak happening this summer. I can't explain it but now that it is coming back I don't worry about it. The other day I gave it some extra bonemeal worked in around the base & watered it in. Next spring it should be fine. I have had other clematis do this once in awhile but never Sunset so I believe it was just something odd this summer. All clematis seem to do good the spring after a brown summer if I cut them back.