I don't know what Sandisk mean by
'it takes 2V' because a USB port outputs 5V.
Let me put it this way - if an AC-USB adaptor was going to fry it due to a difference in voltage then so would plugging it into any USB port, which clearly isn't the case.
And, yes, any AC adaptor with a USB port on it is merely acting as a converter to give you a USB port that's separate from a computer.
It should be identical - apart, obviously, from the fact you can't transfer data across it...
However, I'm going to be a bit of a git at this point and say that should you follow my advice and your Clip dies, I accept no blame. That anything you do, you do so yourself knowing that it's your responsibility.
But, that aside, if I had a Sansa Clip, I'd buy an AC-USB adapter.
edit: in
the user manual, it says:
'You can charge your Sansa® player through the USB port of your computer, a car adapter (not included) USB port, or AC adapter (not included) USB port.'
and
'The battery can also be charged using an AC adapter (not included) or a car adapter (not included) and you will be able to play your device while charging with one of the adapters.'
So, yeah, I'm certain it'll be fine with an adaptor.