Supreme Diana (4)
I believe the answer is:
ross
'supreme diana' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ross that I've seen before include "-- Dependency; -- Sea" , "Wye town; Antarctic sea" , "Jonathan - - (TV)" , "-- And Cromarty, former county of Scotland" , "US singer Diana" .)