A royal boy revealing crime (5)
I believe the answer is:
arson
'crime' is the definition.
(crime of illegally starting a fire)
'a royal boy' is the wordplay.
'royal' becomes 'r' (common abbreviation e.g. RM - Royal Marine).
'boy' becomes 'son' (male child).
'a'+'r'+'son'='ARSON'
'revealing' acts as a link.
(Other definitions for arson that I've seen before include "serious offence" , "It may get you in court" , "Crime of fire setting" , "major crime" , "Malicious crime" .)