Ruler already nasty, to an extent (6)
I believe the answer is:
Here is my best explanation:
'ruler' is the definition.
(dynast is a kind of ruler)
'already nasty to an extent' is the subsidiary indication.
'to an extent' indicates the answer is hidden within the clue.
'dynast' can be found hidden inside 'already nasty'.
(Other definitions for dynast that I've seen before include "One ruling", "Powerful hereditary ruler", "Ruler", "Member of a hereditary ruling family" and "Hereditary ruler".)