Crab, perhaps, unable to be cracked (6)
I believe the answer is:
nebula
'crab perhaps' is the definition.
(as in the Crab Nebula in our solar system)
'unable to be cracked' is the wordplay.
'to be cracked' indicates anagramming the letters.
'unable' is an anagram of 'NEBULA'.
(Other definitions for nebula that I've seen before include "Cloud of gas and dust in outer space" , "Gas/dust cloud in space" , "Interstellar gas cloud" , "Misty appearance in the heavens" , "Unable to be like a distant galaxy" .)