Clue: Literary Romantic hero briefly used to be the king (7)
Answer: WERTHER
Explanation: Definition: Literary Romantic hero
 "were"(used to be) **minus ****its last letter**(briefly …) + THE + R(abbrev. for "Rex"/the reigning king).
Defn: Fictional hero in "The Sorrows of Young Werther", written by Goethe, and which influenced the Romantic movement.