Longest mountain range (5)
I believe the answer is:
andes
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">longest mountain range</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for andes that I've seen before include "Great mountains of western S America" , "World's longest mountain range" , "Aconcagua the highest in this range" , "Mountains in west" , "Mountains for the Danes, oddly" .)