Serve as a go-between (6)
I believe the answer is:
liaise
'serve as a go-between' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for liaise that I've seen before include "Form a link (with)" , "Keep in touch to co-operate on something" , "Mutually cooperate" , "Conduct information between two groups" , "To maintain contact" .)