Pressure lines (7)
I believe the answer is:
isobars
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">pressure lines</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for isobars that I've seen before include "Lines of equal atmospheric pressure" , "can show what pressure folk are under" , "Air boss (anag.)" , "What may be featured on map" , "Lines on a weather map" .)