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