Alien supporting proposal for Mars, maybe (6)
I believe the answer is:
planet
'mars maybe' is the definition.
(Mars is an example)
'alien supporting proposal' is the wordplay.
'alien' becomes 'et' (extra terrestrial).
'supporting' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'proposal' becomes 'plan' (I've seen this before).
'et' put after 'plan' is 'PLANET'.
'for' acts as a link.
(Other definitions for planet that I've seen before include "Orbiting body in the heavens" , "Earth, for instance" , "Large celestial body" , "Mercury?" , "Eg, a ruby" .)