Ruler taking over northerly part (8)
I believe the answer is:
governor
'ruler' is the definition.
(I've seen this before)
'taking over northerly part' is the wordplay.
'part' indicates the answer is hidden within the clue.
'GOVERNOR' is hidden within 'taking over northerly'.
(Other definitions for governor that I've seen before include "person put in charge" , "Regulator; father" , "State ruler" , "Person in charge of colony, prison etc" , "Ron, go rev up the state head" .)