Disturbance in the earth's field (8,5)
I believe the answer is:
magnetic storm
'disturbance in the earth's field' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for magnetic storm that I've seen before include "Disturbance caused by solar flares" , "Disturbance in the earth's field" , "Violent weather" , "Disturbance affecting compasses" .)