Old boy with son backing antisocial youth (5)
I believe the answer is:
yobbo
'antisocial youth' is the definition.
(yob can mean an antisocial youth)
'old boy with son backing' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'with' is a charade indicator (letters next to each other).
'son' becomes 'boy' (synonyms).
'backing' is a reversal indicator.
'ob'+'boy'='obboy'
'obboy' in reverse letter order is 'YOBBO'.
(Other definitions for yobbo that I've seen before include "Thug" , "Surly youth" , "Young troublemaker" , "Tough bloke" , "Rude and aggressive youth" .)