Mars or Mercury (6)
I believe the answer is:
planet
This clue is a double definition.
'mars' is the first definition.
(thesaurus)
'mercury' is the second definition.
(thesaurus)
(Other definitions for planet that I've seen before include "World" , "Sun's orbiter" , "Venus, for example" , "Celestial body that revolves around the sun" , "one of seven in suite" .)