Scheme with alien world (6)
I believe the answer is:
planet
'world' is the definition.
(I've seen this before)
'scheme with alien' is the wordplay.
'scheme' becomes 'plan' (I've seen this before).
'with' is a charade indicator (letters next to each other).
'alien' becomes 'et' (extra terrestrial).
'plan'+'et'='PLANET'
(Other definitions for planet that I've seen before include "Heavenly (or earthly!) body" , "Mercury?" , "Celestial body orbiting round a star" , "Sun's orbiter" , "Eg, Mars or Pluto" .)