The crime of a hard-hearted boy (5)
I believe the answer is:
arson
'the crime' is the definition.
(crime of illegally starting a fire)
'a hard-hearted boy' is the wordplay.
'hearted' means to look at the middle letters.
'boy' becomes 'son' (synonyms).
The middle of 'hard' is 'ar'.
'ar'+'son'='ARSON'
'of' is the link.
(Other definitions for arson that I've seen before include "Criminal pyromania" , "Sonar (anag.)" , "Malicious burning to destroy property" , "Fire requiring forensic study" , "It may get you in court" .)