Not exactly a clip-on mic, but a Schoeps hypercardioid on an H-clamp just works perfectly on my bass. Very feedback resistant, delivers a fat tone that is true to the sound of my instrument. Usually, I mix in just a little bit of my Schertler stat B (single element), which gives the tone a bit more "bite," if that makes any sense. My Grace Felix, finally, is an essential part of my setup, and I send either the mix of the mic and the pickup or the mic alone to the FoH, and/or the mix to my active cab (a Schertler Jam 150 extension, soon to be replaced by the even better Schertler Side, on a pole). This combination isn't cheap, though, but you get what you pay for, and then some!
Peace,
Vincent