Evidence of storm easing when energy is lost (9)
I believe the answer is:
lightning
'evidence of storm' is the definition.
(lightning occurs in a storm)
I don't understand the remainder of the clue.
(Other definitions for lightning that I've seen before include "Flash in the sky" , "Precaution against electrical storms" , "extremely fast?" , "Discharge of electricity in the atmosphere" , "Atmospheric electricity" .)