whats the problem i dont exacly get the question
if ur bolt broke when u where tightning it some of the bolt should b sticking off and u use a drill and tighten it on the screw and then u twist the crew out with the drill
if u have a dremal use it to make the hole into a flat head screw just use a cut off wheel and go straight down on it
only problem with that is ur clamp will have two lines in it