At a loose end's going to the mountains (5)
I believe the answer is:
andes
'the mountains' is the definition.
(I've seen this before)
'at a loose end's' is the wordplay.
'at' says to put letters next to each other.
'loose' indicates an anagram (letters are loosened from their original positions).
'ends' anagrammed gives 'ndes'.
'a' put next to 'ndes' is 'ANDES'.
'going to' acts as a link.
(Other definitions for andes that I've seen before include "Danes seen in mountains" , "Mounts" , "Great S American mountains" , "South American mountain system" , "A geographical feature" .)