Damages the planet (4)
I believe the answer is:
mars
This clue is a double definition.
'damages' is the first definition.
(marring is a kind of damaging)
'the planet' is the second definition.
(Mars is an example)
(Other definitions for mars that I've seen before include "Next planet out" , "Ruins" , "War god - chocolate bar - spoils" , "Damages - planet" , "part of suite" .)