Isthmus (4)
I believe the answer is:
neck
'isthmus' is the definition.
(I've seen this before)
This is the entire clue.
(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" .)