Judge artist performing in silly game (10)
I believe the answer is:
magistrate
'judge' is the definition.
('magistrate' can be a synonym of 'judge')
'artist performing in silly game' is the wordplay.
'performing' indicates anagramming the letters.
'in' is an insertion indicator.
'silly' indicates an anagram.
'artist' is an anagram of 'istrat'.
'game' with letters rearranged gives 'mage'.
'istrat' going inside 'mage' is 'MAGISTRATE'.
(Other definitions for magistrate that I've seen before include "Law enforcer" , "Dispenser of justice" , "one on the bench" , "Court officer" , "Person in court" .)