Organised criminals? Not us, old boy! (3,3)
I believe the answer is:
the mob
'organised criminals?' is the definition.
(I've seen this before)
'not us old boy' is the wordplay.
'not us' becomes 'them' (I've seen this before).
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'them'+'ob'='THE MOB'
(Other definitions for the mob that I've seen before include "gangsters" , "common people" , "lot to which 20 [JAMES GANDOLFINI] in character belonged" , "Organised criminals" .)