So much of a label's meaning is derived from both the context of its use and the experience of the labeler that it's damn near impossible to settle such a debate. In fact, in this particular scenario, the Clash are pretty much interchangeable with the MC5, the Stooges, the Sonics or even - in the hands of the most skilled contrarian - the Beatles and/or Elvis. The best you can hope for is an agreement to disagree. To illustrate my point, I propose that Johnny Cash was far more punk than just about anybody mentioned so far. Further, he was so before such a thing a punk was even on the "scene" radar. What's punker than that? The Clash experimented with lots of different sounds and were largely just ahead of the curve in terms of what the next big thing was. While their s/t was released in the UK the same year as Bollocks, many more bands borrowed from the Clash than from the Sex Pistols, musically speaking and, for better or worse, much more of the Clash's contribution is evident in today's sounds than that of the Pistols'. London Calling incorporated musical styles and production values that other "punk" bands of the time wouldn't have touched. If your criteria is that punk goes against the grain of the times, you have a pretty solid argument for the punk-ness of the Clash. If your criteria has to do with clothes and attitude, that's an entirely different debate, I suppose but it would still be hard to make the argument that the Clash didn't, at least, give it a college try. Personally, - with the exception of Combat Rock - I like the Clash regardless of which categories they might fall into. Additionally, Joe Strummer's solo output serves as evidence that, musically, the Clash were, at the very least, musically capable and, at most, "the only band (punk or otherwise) that matters."
Side note: Generally, these types of arguments degenerate quickly into name calling and sanctimonious name-dropping. I'm new to this forum so it's nice to see some cogent and civilized discourse. Carry on.