Access radeon setting on mac, 3d defective
MacOS does not have Radeon Settings/Catalyst as Windows does. Apple publishes their own drivers through system updates. While Nvidia does offer their own Mac drivers, they often lag far behind the PC counterparts and don't seem to offer any sort of Wattman-equivalent for changing clock speeds.

While there is an app called ATICellerator, it hasn't been updated in almost a decade, and almost certainly won't work with a more modern card, like the one in your iMac.

Conceivably, you could create a custom ROM and reflash your card using Zeus–however, both Zeus and the guides/support for such a modification are relatively limited, especially for a niche card like yours–generally this was used to flash desktop PC cards with Mac firmwares. This is a potentially risky process that may leave you with bricked/dead hardware if you mess up–though you may decide you've got nothing left to lose.

The simplest and best solution is probably to just buy a replacement GPU and replace it following a guide such as the one iFixit publishes. If your card is dying, underclocking it may not save it much longer anyways. If you want to test this, consider installing Windows and attempting to apply a clock change using some sort of reputable app such as Wattman (bundled with the AMD drivers) or MSI Afterburner. You may find that even then, clock changes refuse to stick (even within Windows) for one reason or another. Note that these changes will not pervade through to MacOS.