Longest mountain range (5)
I believe the answer is:
andes
'longest mountain range' is the definition.
(I've seen this in another clue)
This is all the clue.
(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" .)