You may have accidentally damaged the stem so it has wilted. Just cut it off with a pair of sharp scissors or secateurs and leave the other buds alone to open as they may. tis is a group 2 clematis which means it will produce a second flush of lowers in late summer if you remove all the dead heads as they go over.
However, as yours is young and only flowering now, you may not get so many 2nd flush flowers this year. Next spring, don't prune it at all except to remove any obviously dead stems after the new leaf buds start to open. Then give it a good feed of general purpose blood, fish and bone or pelleted chicken manure and a liquid tonic of tomato food for extra flowers. You can also buy specially formulated clematis feed which is designed to promote flower growth. Clematis are hungry plants so don't scimp on the feed but don't feed after June either or you'll get soft sappy growth which won't withstand winter frosts.