Clear is the company that Sprint contracted with to build the WiMAX network. Every 4G area on the Sprint map is Clearwire. You don't have to pay for it, it's included in the $10 fee you already pay (Clear sells their own plans with their own phones/devices too). The phone will just start using it if it grabs a signal. In fact, when it connects, if you go under Settings -> Wireless and Networks -> 4G settings, it'll say Sprint under the network name.
Trust me, I tried it myself. My little hick town in Kansas is in one of the Clear 4G bubbles, but not on the Sprint 4G map. I turned on 4G on the phone, lo and behold I got a message "4G Network Connected", and it says Sprint under the 4G network name. I ran a speed test and it's definitely running on 4G - 1.5-2Mbps up and 9-10Mbps down.