One included in stratagem designed to obtain justice (10)
I believe the answer is:
magistrate
'justice' is the definition.
(synonyms)
'one included in stratagem designed' is the wordplay.
'one' becomes 'I' (Roman numeral).
'included in' is an insertion indicator.
'designed' is an anagram indicator (designed with the letters).
'stratagem' with letters rearranged gives 'magstrate'.
'i' inserted into 'magstrate' is 'MAGISTRATE'.
'to obtain' is the link.
(Other definitions for magistrate that I've seen before include "one on the bench" , "Court head" , "Justice of the peace" , "Judge" , "Law enforcer" .)