It takes me about 70 mins to get to the Doubletree Guest Suites in Tampa from WDW...and that's very close to Tampa International. It's about another 20-30 mins from there to Clearwater. But if you aren't getting off the highway, it will most likely take about an hour and 3/4 I would imagine. I would plan on 2 hrs...just in case you hit traffic.
I have to say that I have never seen traffic move so quickly as in Tampa...bumper to bumper at 80. Really scary.