One's involved in new stratagem to get justice (10)
I believe the answer is:
magistrate
'justice' is the definition.
('magistrate' can be a synonym of 'justice')
'one's involved in new stratagem' is the wordplay.
'one' becomes 'i' (Roman numeral).
'involved in' indicates putting letters inside.
'new' indicates anagramming the letters.
'stratagem' with letters rearranged gives 'magstrate'.
'i' put into 'magstrate' is 'MAGISTRATE'.
'to get' acts as a link.
(Other definitions for magistrate that I've seen before include "Judge" , "Capital of Ethiopia" , "Court head" , "Law enforcer" , "Person in court" .)