Judge, one on circuit's acting for nothing (7)
I believe the answer is:
arbiter
'judge' is the definition.
(I know that arbiter is a type of judge)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for arbiter that I've seen before include "Judge, umpire" , "One judges a disputed issue" , "One must exercise judgment" , "One decides in disputes" , "He'll decide" .)