Possible Clues:

• "Ars ___ artis"
• Part of e.g.
• "Ars — artis" (MGM motto)
• Ex . . . . . . - done as a favour, no charge
• Ex ... means done as favour, not obligation
• ___ Dei
• Ex . . . . . . means free, no obligation
• The 'g' in e.g.
• Ex . . . . . ., optional
• Word in MGM's motto