Anytime! Clearwater has something for everyone and is always warm and sunny. Tampa, where you fly into, is known as the lightning capital of the world. They do get a lot of rain, but only from about 3 to 3:30pm, and then the sun comes back out. Even in December you can lay in the sun in the 75°F weather.
In fact, December and January are my favorite months to go. For one, it sure shakes those winter blues, and for two, that is sand dollar season, when you can walk along the beach and find lots of sand dollars. Dolphins are also a common sight in Clearwater most of the year. With all there is to see and do, you may have to come twice a year. That's what I do!