Distant stars unable to be resolved (6)
I believe the answer is:
nebula
'distant stars' is the definition.
(astronomical term)
'unable to be resolved' is the wordplay.
'to be resolved' indicates an anagram.
'unable' with letters rearranged gives 'NEBULA'.
(Other definitions for nebula that I've seen before include "Cosmic cloud" , "Cloud of gas and dust in outer space" , "Gas/dust cloud in space" , "Unable (anag)" , "Interstellar gas cloud" .)