"Today will be mostly cloudy, it will clear up later in the afternoon." key word "clear up". It can rain, and when it 'clears up' there's no more rain. If it's cloudy and it 'clears up', there's no more clouds. "Cloudy to clear" is not something we would say. It doesn't give enough information, as it sounds like "from.. to", like; "mildly-hot to very hot" (while you're trying to say that it will be cloudy first and then clear).