Oh damn, looks like I need to read up on CPUs again. Sorry for giving out the wrong information.
The usual (and most honest) way to list speed of a multi-core processor is the top speed at which all cores can run simultaneously, while putting out no more heat than the max rated TDP. Almost all modern CPUs can run at lower speeds, to save power, increase battery life, and/or reduce heat loads. Most can run faster under certain conditions.
The common Intel Core CPUs, for instance, have a "Turbo" rating which varies depending on how many cores are in use, and on sufficient power and cooling being available. For instance, an Ivy Bridge quad-core rated at 2.4 GHz might have a Turbo rating of 8/8/9/10 with 100 MHz steps; this means that it can run at up to 3.2 GHz if four or three cores are in use, up to 3.3 GHz with two cores, and 3.4 GHz with only one core used; all this *if* your cooling system can keep the chip from heating up to the point where its internal sensors slow it back down as a safety precaution. Some older and/or mobile chips had more dramatically different Turbo scaling; for instance, some early i7 Nehalem-based mobile quad-cores had scaling of 1/1/6/9 with 133 MHz steps, so it would indeed run considerably faster with two or only one cores than with three or four; but this was the result of internal technological and design limitations, not based on any sort of simple additive logic.
The OP's A6-3400M is a 4-core laptop chip; it's officially rated at 1.4 GHz, with single-stage Turbo up to 2.3 GHz. However, most laptops don't have sufficient cooling to sustain Turbo for long, if at all; and I don't think that line of AMD chips have the ability to Turbo clock individual cores separate from each other, just the whole CPU at once.
Note that none of the above is technically "overclocking"; it's all within the rated and warrantied envelop of the chips. In effect, the simplest sort of what used to be overclocking has been built-in these days. Real overclocking can (sometimes) generate more dramatic results; but tends to reduce component lifetime even under the best of circumstances, and has real risks of damaging your system if you don't know what you are doing, or are pushing the edge.