Boy taken aback by yob's betrayal (4-3)
I believe the answer is:
sell-out
'betrayal' is the definition.
(I've seen this before)
'boy taken aback by yob's' is the wordplay.
'boy' becomes 'les' (short for Leslie).
'taken aback' is a reversal indicator.
'by' says to put letters next to each other.
'yob' becomes 'lout' (both can mean an antisocial young man).
'les' backwards is 'sel'.
'sel'+'lout'='SELL-OUT'
(Other definitions for sell-out that I've seen before include "Event with no further ticket availability" , "Betrayal" , "When all tickets are booked in advance" , "successful show" .)