I've never tried sticking a heatsink on a clockgen (though I've read about it of course) but you should be able to do the same as with anything else.... I used Artic Silver Thermal Epoxy for my Vid card HSF and ramsinks, it works really well (in fact once it's on don't try and ever take it off or you could damage the chip). There's also the old trick of putting thermal paste on the chip, but leaving two corners bare and putting super glue on them.