Transgression of criminal dealer (7)
I believe the answer is:
offence
'transgression' is the definition.
(I know that offence is a type of offence)
'of criminal dealer' is the wordplay.
'criminal dealer' becomes 'fence' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'of'+'fence'='OFFENCE'
Can you help me to learn more?
(Other definitions for offence that I've seen before include "Hurt; military attack" , "Breach of criminal law" , "An illegal act, cause of annoyance" , "Transgression" , "Violation" .)