You can apply to USAFA as a freshmen, whether on scholarship or not. If appointed the scholarship will become void when you show up on I Day at USAFA next summer. If not appointed, than your scholarship stays in place at Clemson.
Just remember what most people call all AFROTC scholarships are 2 + 2. It is in place for the 1st 2 years. If not selected for SFT, chances are high that HQ AFROTC will disenroll your from the program and the last 2 years will not include the scholarship.