Communicate; negotiate (6)
I believe the answer is:
liaise
I believe this clue is a double definition.
'communicate' is the first definition.
(I've seen this before)
'negotiate' is the second definition.
(I know that liaise is a more specific form of the action negotiate)
(Other definitions for liaise that I've seen before include "Mediate" , "Act as a link between groups" , "Mutually cooperate" , "Cooperate on a matter of mutual concern" , "Collaborate" .)