Map possible Martian’s origin (6)
I believe the answer is:
planet
'origin' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'map possible martian's' is the wordplay.
'map' becomes 'plan' (I've seen this before).
'possible martian' becomes 'et' (short for extra-terrestrial).
'plan'+'et'='PLANET'
(Other definitions for planet that I've seen before include "Jupiter is one" , "Substantial heavenly body" , "Venus, for example" , "Earth perhaps" , "Is Pluto one?" .)