Bill is a magistrate (4)
I believe the answer is:
beak
I believe this clue is a double definition.
'bill' is the first definition.
('beak' can be a synonym of 'bill')
'magistrate' is the second definition.
(thesaurus)
'is a' is the link.
(Other definitions for beak that I've seen before include "Horny mouth of a bird" , "Mandible" , "Judge - bill" , "Magistrate (informal)" , "Nose (slang)" .)