Crossword Genius

Crab possibly unable to move (6)

Ross

I believe the answer is:

nebula

I'm a little stuck... Click here to teach me more about this clue!

'crab' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

'unable to move' is the wordplay.
'to move' indicates anagramming the letters.
'unable' with letters rearranged gives 'NEBULA'.

'possibly' acts as a link.

Can you help me to learn more?

(Other definitions for nebula that I've seen before include "Unable to be like a distant galaxy" , "Misty appearance in the heavens" , "Cloud of gas and dust in outer space" , "Bright patch in sky from distant galaxy (6)." , "Cloudy mass in the heavens" .)

I've seen this clue in The Guardian.
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