"Ars — artis” (MGM motto) (6)

I believe the answer is:

gratia

(Other definitions for gratia that I've seen before include "Ex . . . . . ., optional" , "An ex . . . . . . payment is given without obligation" , "Ex . . . . . . means free, no obligation" , "Ex . . . . . . - done as a favour, no charge" , "Ex ... means done as favour, not obligation" .)

I've seen this clue in the King Feature Syndicate.

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