Chief has judge succeeded by unknown official (5)
I believe the answer is:
mayor
'official' is the definition.
(I have seen 'Municipal official' mean 'mayor' so perhaps 'official' could also mean 'mayor')
I cannot understand how the rest of the clue works.
(Other definitions for mayor that I've seen before include "Provost, in Scotland" , "Town's chief citizen." , "For example, Henchard" , "Civic leader" , "Municipal official" .)