Alien after map for Mars, say? (6)
I believe the answer is:
planet
'mars say?' is the definition.
(mars is a kind of planet)
'alien after map' is the wordplay.
'alien' becomes 'et' (extra terrestrial).
'after' says to put letters next to each other.
'map' becomes 'plan' (I've seen this before).
'et' put after 'plan' is 'PLANET'.
'for' is the link.
(Other definitions for planet that I've seen before include "Eg, Mars or Pluto" , "Sun's orbiter" , "Large body orbiting sun" , "Large satellite of star" , "Mars or Uranus" .)