Ferdinand Mount does a fairly comprehensive demolition job on Clarke in The Telegraph today, following Clarke's recantation on the euro and much more besides.
From this account, it is pretty well a "given" that Clarke's motivation is less than sincere, his recantation being seen simply as a rather transparent ploy to clear the way for his election bid as Tory leader. "I am afraid nobody is fooled," writes Mount. "One cannot help recalling the heckler who replied to Harold Wilson's rhetorical question 'Why am I standing up for the Royal Navy?' with the words 'Because you're in Chatham'." "Ken Clarke is a decent bloke… and he might make a splendid President of France. I just don't think he's one for us," Mount concludes.
I think the most damning comment I've heard about Clarke actually came from an MP of my acquaintance who, on hearing of his leadership ambitions, wailed, "but he isn't even a Conservative!"