What does the cryptic crossword clue "Crab, perhaps, unable to be cracked (6)" mean?

The definition is "Crab, perhaps,". The wordplay uses an anagram. anagram of UNABLE = NEBULA; definition: "Crab, perhaps," The answer is NEBULA.

Crab, perhaps, unable to be cracked (6)

Times Cryptic #28909 | 10 Across | 2024-05-06 flower