I'm to act as a go-between next (9)
I believe the answer is:
immediate
'next' is the definition.
(thesaurus)
'i'm to act as a go-between' is the wordplay.
'to act as a go-between' becomes 'mediate' (I've seen this in another clue).
'im'+'mediate'='IMMEDIATE'
(Other definitions for immediate that I've seen before include "Prompt,straightaway" , "Nearest" , "Direct" , "Involving no delay" , "Instantaneous" .)