Sketch alien's heavenly body (6)
I believe the answer is:
planet
'heavenly body' is the definition.
(planet is a type of heavenly body)
'sketch alien's' is the wordplay.
'sketch' becomes 'plan' (I've seen this in another clue).
'alien' becomes 'et' (extra terrestrial).
'plan'+'et'='PLANET'
(Other definitions for planet that I've seen before include "Sun's orbiter" , "Heavenly body moving round the sun" , "Orbiting heavenly body" , "Heavenly (or earthly!) body" , "Large celestial body" .)