One day, wise guy! (7)
I believe the answer is:
solomon
'wise guy' is the definition.
(I've seen this in another clue)
'one day' is the wordplay.
'one' becomes 'solo' (I've seen this before).
'day' becomes 'mon' (short for Monday).
'solo'+'mon'='SOLOMON'
(Other definitions for solomon that I've seen before include "Wise old king of Israel" , "Son of King David" , "Sage" , "King of Israel with great wisdom" , "King of Israel noted for his wisdom" .)