Right to stop a boy in crime (5)
I believe the answer is:
arson
'crime' is the definition.
(crime of illegally starting a fire)
'right to stop a boy' is the wordplay.
'right' becomes 'r' (common abbreviation).
'to stop' means one lot of letters goes inside another.
'boy' becomes 'son' (synonyms).
'a'+'son'='ason'
'r' inserted within 'ason' is 'ARSON'.
'in' is the link.
(Other definitions for arson that I've seen before include "Crime of fire raising" , "Incendiary crime" , "Felonious burning of property" , "Torching" , "Fire requiring forensic study" .)