One day he was wise (7)
I believe the answer is:
solomon
'he was wise' is the definition.
(King Solomon, wise and wealthy biblical King)
'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 "Wise old king of Israel" , "Sage" , "Wise man" , "Old king of Israel famous for wisdom and magnificence" , "King known for wisdom" .)