Astronomical cloud (6)
I believe the answer is:
nebula
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">astronomical cloud</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)