Spoils of war? (4)
I believe the answer is:
Here is my best explanation:
I believe this clue is a double definition.
'spoils' is the first definition.
('mar' can be a synonym of 'spoil')
'war?' is the second definition.
(I've seen this before)
'of' is the link.
(Other definitions for mars that I've seen before include "Heavenly place", "Ruins", "Roman god of War", "God of war", "world", "Next planet out", "Spoils - heavenly body", "Fourth rock from the Sun", "Roman war god; planet" and "Planet; Roman god".)