Democrat replacing nasty hereditary ruler (6)
I believe the answer is:
dynast
'hereditary ruler' is the definition.
(I've seen this before)
'democrat replacing nasty' is the wordplay.
'democrat' becomes 'd'.
'replacing' indicates anagramming the letters (re-placing: placing the letters again).
'nasty' is an anagram of 'ynast'.
'd'+'ynast'='DYNAST'
(Other definitions for dynast that I've seen before include "One ruling" , "Member of a hereditary ruling family" , "Powerful hereditary ruler" .)