He ruled alone one day (7)
I believe the answer is:
solomon
'he ruled' is the definition.
The definition suggests a singular noun which matches the answer.
'alone one day' is the wordplay.
'alone' becomes 'solo' (as in a musical solo).
'one day' becomes 'mon' (abbreviation for Monday).
'solo'+'mon'='SOLOMON'
Can you help me to learn more?
(Other definitions for solomon that I've seen before include "King of Israel with great wisdom" , "Short-lived" , "King known for wisdom" , "Wise man" , "wise guy" .)