Thin cord (6)
I believe the answer is:
string
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">thin cord</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 string that I've seen before include "It's difficult to know how long a piece of it is" , "How long could a piece of it be?" , "Group of horses" , "Collectively, the racehorses trained at one stable" , "Narrow cord or kind of mesh vest" .)