This is one of just two tracks that drummer Pete Howard (who replaced Topper Headon in 1983 after the latter was fired for drug addiction problems) actually played on in the Cut the Crap sessions - the other being fellow B-side "Sex Mad Roar." These two tracks were also the only two that the full band played on together in the studio - the rest were recorded in bits, mostly using session musicians and drum machines.
"Do It Now" was never played live by the Clash, and in fact only got a full LP release on the 2000 CD reissue of Cut the Crap. For many years it was only available as the B-side to the "This Is England" single.
It's also not on any rarities compilations (such as Super Black Market Clash) due to the band making a decision to basically ignore the period of The Clash from after Mick Jones and Topper Headon left in every retrospective. Notice how this period isn't even mentioned in the Westway to the World documentary, for example.
The Naughty by Nature hit "O.P.P." doesn't have any curse words, but many oversensitive radio stations played a "clean" version with the word "kitten" edited out, surely the first time that word was censured.