Similar idea to over/under clocking. Use AMD Wattman (or download a utility like MSI afterburner) and start incrementally reducing the GPU voltage, waiting a bit after each increment (setting voltage to 1100 mV might be a good place to start). Run a benchmark like unigine valley in the background while doing this, and watch for any signs of instability (graphical artifacts, driver crashes, etc). One you see signs of instability, go back to the previous voltage and let that run for a while while still observing. If you start seeing instability again, increase voltage a bit further and repeat, if not you're good. IIRC in some cases instability will only manifest itself after the card has heated up.
If you're using Wattman, adjust the voltage for P states 6 and 7. You also want to monitor to see that voltage is actually going down as you are setting it lower. I think you can monitor this in Afterburner, but if you're using Wattman you may need to download another monitoring utility like GPU-Z. In wattman, if you reach the point where you've reduced the GPU voltage to the memory voltage, you'll have to start reducing each (by the same amount) to continue lowering voltage.
You don't have to worry about ruining your card. Worst case is you'll end up with some crashes/instability, which you can always fix by reverting to stock settings.
Can see a couple example screenshots of Wattman here:
https://wccftech.com/article/radeon-rx-480-reducing-voltage-increasing-efficiency/