Oh shit. I forgot that some of the best Clash songs were covers.
The Clash - I fought the Law - Bobby Fuller
The Clash - Revolution Rock - Danny Ray
The Clash - Police and Thieves - Junior Murvin
The Clash - Junco Partner - It's a folk song, but various jazz musicians in the 30's and Joe Strummers R&B band before the Clash, The 101er's.
There are probably more that I can't remember. The Clash used to do a lot of covers of really, really old reggae songs.
And Joe Strummer used to do a great live cover of The Pogues's "If I should Fall From Grace with God."