Astronomical cloud (6)
I believe the answer is:
nebula
'astronomical cloud' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for nebula that I've seen before include "Mass of interstellar gas" , "Unable (anag)" , "Bright patch in sky from distant galaxy (6)." , "Misty appearance in the heavens" , "Cloudy mass in the heavens" .)