Interstellar cloud (6)
I believe the answer is:
nebula
'interstellar 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 "Cloudy mass in the heavens" , "Unable to find patch in the sky" , "Bright patch in sky caused by distant galaxy" , "Mass of interstellar gas" , "Unable to be like a distant galaxy" .)