Trace son mistreated by forebear (8)
I believe the answer is:
ancestor
'forebear' is the definition.
(thesaurus)
'trace son mistreated' is the wordplay.
'mistreated' indicates anagramming the letters.
'trace'+'son'='traceson'
'traceson' is an anagram of 'ANCESTOR'.
'by' acts as a link.
(Other definitions for ancestor that I've seen before include "Adam was our first one" , "Person from whom one is descended" , "Antecedent, forebear" , "Forefather" , "One of one's forebears" .)