Turn to prime sort of judge (6)
I believe the answer is:
umpire
'judge' is the definition.
(I know that judge is a is a more specific form of the action of umpire)
'turn to prime sort' is the wordplay.
'turn' becomes 'U' (a 'U-turn' is a kind of turn).
'to' is a charade indicator (letters next to each other).
'sort' indicates anagramming the letters.
'prime' is an anagram of 'mpire'.
'u'+'mpire'='UMPIRE'
'of' acts as a link.
(Other definitions for umpire that I've seen before include "Person enforcing rules" , "The ref in cricket match" , "Kind of referee as in cricket" , "Sports arbiter" , "Adjudicator in games such as cricket, tennis etc" .)