Narrow joining strip of land (7)
I believe the answer is:
isthmus
'narrow joining strip of land' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for isthmus that I've seen before include "Narrow strip of land joining two larger entities" , "Narrow neck of land connecting two larger portions" , "This sum (anag)" , "Narrow strip of land in sea" , "Narrow stretch of land joining two larger masses" .)