In the IDE you can set the clock to either external (the frequency has to match the actual crystal used and is used by the compiler for timing calculations), or internal (and there you can set the actual speed), then burn it to the chip (set "fuses").
Not sure if/how this works for an Arduino, as the development board itself may get in the way. But for low power use, you have to get rid of that part anyway. Go bare bones.
A way to save even more power is to switch to a different processor, such as the ATtiny85 or 84a. Whether that works depends on your actual project.