One's referee is closest (9)
I believe the answer is:
immediate
'closest' is the definition.
(as in your immediate family)
'one's referee' is the wordplay.
'one's' becomes 'I'm' ('one is' is a formal way of saying 'I am').
'referee' becomes 'mediate' (referee can mean to mediate or moderate).
'im'+'mediate'='IMMEDIATE'
'is' is the link.
(Other definitions for immediate that I've seen before include "Instantaneous" , "Nearest" , "Prompt, straightaway" , "Prompt,straightaway" , "Occurring without delay" .)