Ruler already nasty, to an extent (6)
I believe the answer is:
dynast
'ruler' is the definition.
(dynast is a kind of ruler)
'already nasty to an extent' is the wordplay.
'to an extent' says the answer is hidden in the clue.
'DYNAST' is hidden within 'already nasty'.
(Other definitions for dynast that I've seen before include "One ruling" , "Member of a hereditary ruling family" , "Powerful hereditary ruler" .)