Band making one group (7,4)
I believe the answer is:
wedding ring
'band' is the definition.
(I know that wedding ring is a type of band)
'one group' is the wordplay.
'one' becomes 'wedding' (I can't justify this - if you can you should believe this answer much more).
'group' becomes 'ring' (a ring is a criminal organisation).
'wedding'+'ring'='WEDDING-RING'
'making' acts as a link.
Can you help me to learn more?
(Other definitions for wedding ring that I've seen before include "Left-hand, fourth-finger band" , "symbol of commitment to union" .)