You'll have to open another iCloud account - you can't share access to just part of the email account.
Go to http://appleid.apple.com and create a new ID - you will need a different non-Apple email address to do this.
Sign in with it in System Preferences>Mail, Contacts and Calendars and enable 'Mail' in the checklist there. You should be asked to create a new @icloud.com address.
This account will then appear alongside your present one in Mail. Your lawyer can access it in the usual way - if he doesn't have a Mac he can set the account up manually using the server settings here: