Line on a weather map (6)
I believe the answer is:
isobar
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">line on a weather map</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 isobar that I've seen before include "Map-line showing places with equal air pressure" , "Equal-pressure line" , "Line on map connecting places of equal atmospheric pressure" , "Weather-map line" , "Line on map showing atmospheric pressure" .)