Cloud in space (6)
I believe the answer is:
nebula
'cloud in space' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for nebula that I've seen before include "Mist of gas or stars in the night sky" , "Hazy light; a slight cloudiness" , "Bright patch in sky caused by distant galaxy" , "Diffuse cloud of particles and gas in space" , "Cloudy mass in the heavens" .)