If you need to enable Clutter for a single user (or even the whole organisation!) on O365, then it can be done very easily with PowerShell:

# Connect to O365
$UserCredential = Get-Credential
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://outlook.office365.com/powershell-liveid/ -Credential $UserCredential -Authentication Basic -AllowRedirection
Import-PSSession $Session

# Enable Clutter For A Single User
Set-Clutter -Identity emailaddress@domain.co.uk -Enable $true

# Enable Clutter For All Users
Get-Mailbox -ResultSize Unlimited | Set-Clutter -Enable $true

Easy peasy 🙂