Narrow joining strip of land (7)
I believe the answer is:
isthmus
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">narrow joining strip of land</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 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" .)