Next, I'm to act as arbitrator (9)
I believe the answer is:
immediate
'next' is the definition.
(thesaurus)
'i'm to act as arbitrator' is the wordplay.
'to act as arbitrator' becomes 'mediate' (I've seen this in another clue).
'im'+'mediate'='IMMEDIATE'
(Other definitions for immediate that I've seen before include "Most urgent; current" , "Occurring without delay" , "Instant" , "Prompt, straightaway" , "Involving no delay" .)