Subordinate as slave endlessly mistreated (6)
I believe the answer is:
vassal
'subordinate' is the definition.
(thesaurus)
'as slave endlessly mistreated' is the wordplay.
'endlessly' means to remove the last letter.
'mistreated' is an anagram indicator.
'as'+'slave'='asslave'
'asslave' with its final letter taken away is 'asslav'.
'asslav' anagrammed gives 'VASSAL'.
(Other definitions for vassal that I've seen before include "He owed allegiance and service to a feudal lord" , "One owing fealty" , "Subordinate person or country" , "Serf, humble servant of old" , "One owed allegiance to a feudal lord" .)