Criminal to John's rear is old con (6,5)
I believe the answer is:
trojan horse
'old con' is the definition.
(deception used in the Trojan War)
'criminal to john's rear' is the wordplay.
'criminal' is an anagram indicator.
'to'+'johns'+'rear'='tojohnsrear'
'tojohnsrear' anagrammed gives 'TROJAN HORSE'.
'is' is the link.
(Other definitions for trojan horse that I've seen before include "Investment aid" , "Something introduced to defeat from within" , "admitted losing the war" , "Malicious program" , "Subversive means of attack" .)