Narrow strip of land connecting two larger land areas, such as the — of Panama (7)
I believe the answer is:
isthmus
(Other definitions for isthmus that I've seen before include "Narrow stretch of land joining two larger masses" , "Strip of land flanked by sea" , "Narrow strip of land joining two larger entities" , "eg Corinth, Panama" , "Narrow strip of land with water on two sides" .)