Wise men, at rest, upset judge (10)
I believe the answer is:
magistrate
'judge' is the definition.
(synonyms)
'wise men at rest upset' is the wordplay.
'wise men' becomes 'magi' (the three wise men from the nativity).
'upset' indicates anagramming the letters.
'at'+'rest'='atrest'
'atrest' with letters rearranged gives 'strate'.
'magi'+'strate'='MAGISTRATE'
(Other definitions for magistrate that I've seen before include "Justice of the peace" , "JP" , "Law enforcer" , "Dispenser of justice" , "Person in court" .)