Curved line (3)
I believe the answer is:
arc
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">curved line</span>' is the definition.<br/ ><span class="explanation-format__explanation">(an arc is a part of a curved line)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for arc that I've seen before include "Rainbow, e.g." , "Bow" , "Semi-circle" , "Something used to weld" , "Part of a circular line" .)