To turn Clutter off log into Outlook for the web , On the top right corner of the page, go to Settings > Your app settings. On the left navigation bar choose Clutter. Un-click the check box next to 'Separate items identified as clutter and click Save. Note: The Clutter folder remains in Outlook after you turn off the Clutter feature. If you don’t want the folder listed under your Mailbox, move the emails from Clutter to other folders, as appropriate, and then right-click the Clutter folder and choose Delete Folder.
For more information please visit Microsoft's Use Clutter to sort low priority messages in Outlook page.