Disturbance in the earth's field (8,5)
I believe the answer is:
magnetic storm
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">disturbance in the earth's field</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)