Contact between groups with back trouble is working (7)
I believe the answer is:
liaison
'contact between groups' is the definition.
(I've seen this before)
'back trouble is working' is the wordplay.
'back' says the letters should be written backwards.
'trouble' becomes 'ail' ('ail' can be a synonym of 'trouble').
'working' becomes 'on'.
'ail' in reverse letter order is 'lia'.
'lia'+'is'+'on'='LIAISON'
'with' is the link.
(Other definitions for liaison that I've seen before include "Secret love affair" , "Secret love-affair" , "Usually secretive or illicit relationship" , "Amour; communication" , "carryon" .)