Weather line (6)
I believe the answer is:
isobar
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">weather line</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for isobar that I've seen before include "curve on map" , "Line on map showing atmospheric pressure" , "Pressure line on a weather map" , "Weather map feature" , "Equal-pressure line" .)