Literary Romantic hero briefly used to be the king (7)
I believe the answer is:
werther
'literary romantic hero' is the definition.
(from the novel The Sorrows of Young Werther)
'briefly used to be the king' is the wordplay.
'briefly' means to remove the last letter.
'used to be' becomes 'were' (eg 'you were...' means 'you used to be...').
'king' becomes 'R' (abbreviation of Latin rex).
'were' with its final letter taken off is 'wer'.
'wer'+'the'+'r'='WERTHER'
(Other definitions for werther that I've seen before include "Massenet opera (Goethe)" , "romantic hero?" .)