Intermediary in affair (7)
I believe the answer is:
liaison
I believe this clue is a double definition.
'intermediary' is the first definition.
(thesaurus)
'affair' is the second definition.
('liaison' can be a synonym of 'affair')
'in' acts as a link.
(Other definitions for liaison that I've seen before include "Naughty affair?" , "Amour; communication" , "Communication between groups or units" , "Secret love affair" , "Co-operation; love affair" .)