Sack impure judge (6)
I believe the answer is:
umpire
'judge' is the definition.
(I know that umpire is a more specific form of the action judge)
'sack impure' is the wordplay.
'sack' indicates anagramming the letters (sack can mean destroy or spoil).
'impure' is an anagram of 'UMPIRE'.
(Other definitions for umpire that I've seen before include "A field judge in circket" , "Arbiter of cricket match" , "Person enforcing rules" , "The ref in cricket match" , "Kind of referee as in cricket" .)