Supremo for D-Day operation? (8)
I believe the answer is:
overlord
I believe this is a double definition.
'supremo' is the first definition.
'd-day operation?' is the second definition.
(D-Day was codenamed Operation Overlord)
'for' is the link.
(Other definitions for overlord that I've seen before include "Ruler, D-day Operation" , "top man!" , "Feudal ruler" , "Wartime operation" , "Supreme ruler" .)