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' shows that the letters should be reversed in order.
'by' means one lot of letters go next to another.
'yob' becomes 'lout' (I've seen this before).
'les' written backwards gives '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" .)