Clue: With a better argument, American Mancini ultimately pens 'keep Moon River' (1,8)
Answer: A FORTIORI
Explanation: Definition: With a better argument
A (American) + ultimate letter of [Mancin]I; around FORT="keep" + IO="Moon" of Jupiter + R (River)
 the song *Moon River* was composed by the American Henry Mancini