Operation Ivy bit the Clash's act. So what does that mean? I think it means your equation is wrong.
I think Rancid after their third album bit the Clash much more than anything OpIvy every did.
Besides, the Clash aren't ska-punk in the sense of bands like Less Than Jake, the Bosstones and Reel Big Fish. Yes, the Clash were a punk band who had lots of ska influences in some of their songs, but I wouldn't consider their sound ska-punk. I mean, is "Should I Stay or Should I Go" ska-punk? Or "Train in Vain"? What about "Rock the Casbah"?
I think not.
The Clash weren't ska-punk band, they wera a punk band who played some ska.