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 in another clue)
'at a loose end's' is the wordplay.
'at' is a charade indicator (letters next to each other).
'loose' indicates anagramming the letters (letters are loosened from their original positions).
'ends' anagrammed gives 'ndes'.
'a'+'ndes' is 'ANDES'.
'going to' is the 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" .)