Legendary monarch, king in a day (6)
I believe the answer is:
arthur
'legendary monarch' is the definition.
(King Arthur)
'king in a day' is the wordplay.
'king' becomes 'R' (abbreviation of Latin rex).
'in' is an insertion indicator.
'day' becomes 'thur' (abbreviation for Thursday).
'a'+'thur'='athur'
'r' going within 'athur' is 'ARTHUR'.
(Other definitions for arthur that I've seen before include "Owner of Excalibur" , "Chester in the eighteen hundreds?" , "Legendary king of Camelot." , "Dudley Moore film" , "Sir --- Conan Doyle" .)