Yup, its about 45 minutes to Clearwater beach from Busch, but might I ask why only half a day? There is plenty to be done and you might as well get your admissions worth.
Besides that fact if you are going during the summer, spend half the day at Busch and then head over to a beach you might have a tad bit of trouble finding a place to park around that time of the day, just giving you a warning on that one.