A criminal organisation? Old boy’s supporting them (3,3)
I believe the answer is:
the mob
'a criminal organisation?' is the definition.
(the mob can mean the mafia)
'old boy's supporting them' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'ob' after 'them' is 'THE MOB'.
(Other definitions for the mob that I've seen before include "gangsters" , "lot to which 20 [JAMES GANDOLFINI] in character belonged" , "common people" , "Organised criminals" .)