Sober lad goes wild — he can't take defeat (3,5)
I believe the answer is:
bad loser
'he can't take defeat' is the definition.
(a bad loser reacts badly to defeat)
'sober lad goes wild' is the wordplay.
'goes wild' is an anagram indicator.
'sober'+'lad'='soberlad'
'soberlad' is an anagram of 'BAD LOSER'.
(Other definitions for bad loser that I've seen before include "Person who cannot accept defeat with grace" , "Person who can't accept defeat gracefully" , "He lacks grace in defeat" .)