Diffuse nebula lacking in strength (6)

I believe the answer is:

unable

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'diffuse' is the definition.
Both the definition and answer are adjectives. Maybe they are linked in a way I don't understand?

'nebula lacking in strength' is the wordplay.
'nebula' anagrams to 'UNABLE'.
But, I'm not clear how this is indicated.

Can you help me to learn more?

(Other definitions for unable that I've seen before include "Nebula (anag)" , "Not fit or competent" , "Not in a position (to)" , "Lacking the necessary skills" , "Incapable" .)

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