A while back, I covered how to delete of all of your iCloud Keychain data from Apple’s servers if you need to, but how can you remove just one saved login? It’s simple, and you can do it from either your Mac or from an iOS device. Since the data syncs, you’ll only need to trash the info from one of them, and the change should propagate to all your devices.
So to clear out a password from your Mac, open Safari and pick Safari> Preferences.
Choose the Passwords tab, and then select the item you want from the list and click "Remove."
From this pane, you can also show what the password is for a particular site if you click the "Show passwords for selected websites" checkbox in the lower-left corner, but you’ll have to enter your account password in order to do so.
OK, now how do you do that same thing from an iOS device? Easy-peasy, my dear readers. You’ll go to Settings> Safari> Passwords & AutoFill and select "Saved Passwords"…
…then select an item from the list to view its existing password (which’ll require you to enter the device’s passcode), or you can touch the Edit button to remove entries.
Need more help on setting up and using iCloud Keychain? Apple has a ton of information available on their support pages. They’ve also got an entire section on what to do if your Keychain doesn’t seem to be working properly, so if yours is misbehaving, be sure to check it out!