Unable to prevent a blackout? (9)

I believe the answer is:

powerless

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'unable' is the definition.
(similar in meaning)

I don't understand the remainder of the clue.

(Other definitions for powerless that I've seen before include "hamstrung" , "Lacking ability" , "Weak, impotent" , "Without ability" .)

I've seen this clue in The Daily Express.

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