"The Red Planet" (4)
I believe the answer is:
mars
'the red planet' is the definition.
('Mars' can be a synonym of 'red planet')
This is all the clue.
(Other definitions for mars that I've seen before include "RS [RED STAR] (not technically)" , "blights" , "Spoils - heavenly body" , "Famous Texas mission" , "Ruins" .)