Sorcerer arrests first traitor getting justice (10)
I believe the answer is:
magistrate
'justice' is the definition.
(I know that magistrate is a type of justice)
'sorcerer arrests first traitor' is the wordplay.
'sorcerer' becomes 'mage' (I've seen this in another clue).
'arrests' is an insertion indicator (inserted letters are held or arrested).
'first' becomes 'ist' (ist resembles 1st).
'traitor' becomes 'rat' (rat can mean a treacherous person).
'ist'+'rat'='istrat'
'mage' going around 'istrat' is 'MAGISTRATE'.
'getting' acts as a link.
(Other definitions for magistrate that I've seen before include "Court official" , "Capital of Ethiopia" , "Person in court" , "one on the bench" , "Court head" .)