Narrow neck of land (7)
I believe the answer is:
isthmus
I believe this clue is a double definition.
'narrow' is the first definition.
(thesaurus)
'neck of land' is the second definition.
(I've seen this before)
(Other definitions for isthmus that I've seen before include "Narrow stretch of land joining two larger masses" , "This sum (anag)" , "Narrow strip of land connecting two large masses" , "Narrow neck of land connecting two larger portions" , "Narrow linking piece of land" .)