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Tap off the input via a resistor, feed to a cap with the other side of the cap going to ground and run the input and the cap/resistor combination to a differential amplifier.
When the input is at a stable voltage, the two inputs will be the same and the diff amp will be at zero output volts, if the input fluctuates then the diff amp will fluctuate accordingly less the DC value stored in the cap.
Size of resistor and cap depend on the frequency you're measuring.
Actually - rereading the original question I think I've got my wires crossed lol. Damn this lack of sleep.
My idea is to remove the DC offset rather than apply one to an AC input.
My bad.
First off we really need to know why he wants to do this, which may make everything a LOT simpler.
But (as an old idea) he might google "DC restoration" which was used in TV's year ago.