I think your talking about the brass retaining clip that holds the catridge in place. It's function is to hold the catridge in place under water pressure. It can be removed with needle nose pliers. When you re-install the new catridge make sure the 2 little ears on the bottom of the clip are fully inserted through the valve. Sometimes a good amount of force is needed to pull the old cartridge out, but no special tools are required. A trick is to screw in the handle screw and use your channel locks to pull it straight out.

Remove the retaining clip...and use a "moen cartridge puller" if necessary. Moen has manufactured the "puller" for their products to prevent damaging the valve when the cartridge is stuck. erik

thanks tom exactly what i needed to know ,but my troubles run deeper!! find my new post and any help would be appreciated!!