King of Israel one day (7)
I believe the answer is:
solomon
'king of israel' is the definition.
(I've seen this before)
'one day' is the wordplay.
'one' becomes 'solo' (I've seen this before).
'day' becomes 'mon' (abbreviation for Monday).
'solo'+'mon'='SOLOMON'
(Other definitions for solomon that I've seen before include "Son of King David" , "King of Egypt famous for wisdom and magnificence" , "OT king" , "Old king of Israel famous for wisdom and magnificence" , "King known for wisdom" .)