Be a go-between (7)
I believe the answer is:
'be a go-between' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for mediate that I've seen before include "Act as conciliator" , "Act as negotiator or peacemaker" , "Act as middle man" , "ie Tamed (anag.)" , "Intercede" .)