Olympian ruler (4)
I believe the answer is:
zeus
'olympian ruler' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for zeus that I've seen before include "Top deity on Olympus" , "Supreme Greek deity" , "Divine son of Cronos and Rhea" , "Godfather?" , "Supreme god of the ancient Greeks" .)