The fourth planet (4)
I believe the answer is:
mars
'the fourth planet' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for mars that I've seen before include "bar....that's sweet!" , "Roman war god; planet" , "War god - chocolate bar - spoils" , "Fourth rock from the Sun" , "Damages - planet" .)