Isthmus (4)
I believe the answer is:
neck
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">isthmus</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 neck that I've seen before include "Part of the body - cheek" , "Adam's apple location" , "Narrow connecting part - smooch" , "A short lead" , "Narrow part of e.g. isthmus" .)