King alone for a day (7)
I believe the answer is:
solomon
'king' is the definition.
(I've seen this before)
'alone for a day' is the wordplay.
'alone' becomes 'solo' (as in a musical solo).
'for' means one lot of letters go next to another.
'a day' becomes 'mon' (abbreviation for Monday).
'solo'+'mon'='SOLOMON'
(Other definitions for solomon that I've seen before include "King known for wisdom" , "King of Israel with great wisdom" , "OT king" , "Son of King David" , "Biblical wise king" .)