Judge's arrival about to take effect (7)
I believe the answer is:
arbiter
'judge's' is the definition.
(I've seen this before)
'arrival about to take effect' is the wordplay.
'arrival' becomes 'arr' (abbreviation used in transport).
'about' means one lot of letters goes inside another.
'to take effect' becomes 'bite' (I've seen this before).
'arr' enclosing 'bite' is 'ARBITER'.
(Other definitions for arbiter that I've seen before include "Judge - mediator" , "Referee or judge of rarebit" , "One appointed to settle a dispute" , "One has the authority to settle disputes" , "Rear bit (anag.)" .)