By Ben Marks — On December 11, 1968, Eric Clapton stepped onto a low stage inside Intertel (V.T.R. Services) Studio north of London to play a few tunes with John Lennon and Keith Richards; the occasion was the taping of “The Rolling Stones Rock and Roll Circus” for the BBC. At the time, Clapton was the most revered electric guitarist in rock, even amid the incendiary ascendancy of Jimi Hendrix. A few weeks earlier, Clapton had performed his last two shows as a member of Cream at the prestigious Royal...