Crossword Genius

Demanding type of test for fuelless energy (9)

Ross

I believe the answer is:

stressful

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'demanding' is the definition.

I don't understand how the rest of the clue works.

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(Other definitions for stressful that I've seen before include "Causing mental tension" , "under great pressure" , "Very hard on the nerves" , "tense" , "Creating tension" .)

I've seen this clue in The Guardian.
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