Long narrow mark (4)
I believe the answer is:
line
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">long narrow mark</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for line that I've seen before include "Railway track; wrinkle" , "column" , "Ancestry" , "job" , "Winter sports slope" .)