Demanding type of test for fuelless energy (9)
I believe the answer is:
stressful
'demanding' is the definition.
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for stressful that I've seen before include "Causing mental tension" , "under great pressure" , "Very hard on the nerves" , "tense" , "Creating tension" .)