Eric Clapton and Robert F. Kennedy Jr. consider themselves rebels in their respective fields -- but those worlds collided in a big way this week in an effort to raise cash ... and a lot of it. The rocker performed at a private show Monday night in L.A. -- where EC plugged in his amp and started strumming at a ritzy estate in Brentwood. No word on who exactly hosted -- but it sounds like the place was packed ... 'cause the show raised $2.2 million.