Historian reveals bad king to us in a different way (9)
I believe the answer is:
herodotus
'historian' is the definition.
(Greek historian)
'bad king to us in a different way' is the wordplay.
'bad king' becomes 'Herod' (King Herod in the Bible).
'in a different way' indicates anagramming the letters (letters ordered a new way).
'to'+'us'='tous'
'tous' is an anagram of 'otus'.
'herod'+'otus'='HERODOTUS'
'reveals' acts as a link.
(Other definitions for herodotus that I've seen before include "Famous Greek" , "Old Greek" , "Ancient Greek historian" , "Historical pioneer" .)