LOS ANGELES (Reuters) - John Lennon held out the promise he could bring out more musical greatness in legendary guitarist Eric Clapton in a letter that could fetch as much as $30,000 when it is sold at auction next month, the organizers of the sale said on Monday. The signed, hand-written letter by the Beatle, who died in 1980 at the age of 40, is one of a selection from some of the world's great musicians that will go under the hammer in Los Angeles at the Profiles in History auction on December 18. ...