Touching lines (8)
I believe the answer is:
tangents
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">touching lines</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 tangents that I've seen before include "Straight lines - ten gnats (anag)" , "Straight lines that touch but don't intersect a curve" , "Straight lines that touche curves" , "Straight lines that touch curves" , "Touching lines" .)