If you clamp an input, you add a circuit that will clip the input range. This is something done to protect from overvoltage.

In your case, you have an AC signal and wants to measure this signal using a DC-only ADC. This is a completely different issue.

If the ADC is very fast, in relation to the AC frequency, then you can make many samples and locate the maxima and minima. You would probably have to add in a DC offset to avoid negative voltages to the ADC - few internal ADC can handle negative voltages. For even better precision, you can use all measurement values and a bit of statistical methods to compute the rms voltage and the frequency. This makes your readings less affected by noise or spikes on the input signal.

If the ADC is slow, then you may use some form of low-pass filter before the ADC. Remember that if you send the AC to a rectifier bridge and then have a capacitor, then the voltage over the capacitor will be a ripple with the highest voltage corresponding to the peak voltage of the ADC signal, minus the voltage loss in the rectifier. If the capacitor is lightly loaded, then the ripple will be low. Just note that if the AC voltage drops, then the voltage in the capacitor will be too high, unless there is a load somewhere that discharges the capacitor.