Go-between (12)
(Other definitions for intermediary that I've seen before include "Go-between" , "Negotiator" , "Agent" .)
(Other definitions for liaison that I've seen before include "Channel of communication" , "Communication between people or organisations" , "Communication; relationship" , "Contact between groups" , "Fling - intrigue" .)
'go-between' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for mediator that I've seen before include "Person working to settle disputes" , "Negotiator between opposing parties" , "One intervening to settle a dispute" , "An honest broker" , "Go-between, negotiator" .)