Line of longitude (8)
I believe the answer is:
meridian
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">line of longitude</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for meridian that I've seen before include "One passing through Greenwich" , "Imaginary great circle on the earth passing through both poles" , "Imaginary line of longitude" , "Imaginary great circle through North and South poles" , "Highest point" .)